enable PIE by default on Linux for full ASLR

Rust already builds all code as position independent by default, so the
linker can be told to build a position independent executable if it's
not disabled with `-C relocation-model=dynamic-no-pic`. Position
independent code does have a significant cost on i686 (not on x86_64 or
ARM) but there's no significant cost to linking code that's already
position independent as a position independent executable.

Address space layout randomization makes exploiting vulnerabilities much
more difficult by providing a statistical defence against an attempt to
find or modify existing code / data. Without ASLR, it's trivial to use a
vulnerability to take over control of the process via return-oriented
programming.

Rust code can be used for return-oriented programming whether it is safe
or unsafe, so even a fully safe application needs to be built as a
position independent executable to defend against vulnerabilities in
unsafe blocks or C libraries.

Sample program:

    extern crate libc;

    use std::mem;

    static mut global: u32 = 5;
    static constant: u32 = 5;
    fn foo() {}

    fn main() {
        let local = 5;
        println!("stack: {}, global: {}, constant: {}, fn: {}, lib fn: {}",
                 &local as *const u32,
                 unsafe { &global as *const u32 },
                 &constant as *const u32,
                 unsafe { mem::transmute::<_, *const ()>(foo) },
                 unsafe { mem::transmute::<_, *const ()>(libc::mprotect) });
    }

Before:

    stack: 0x3ff15eb9f94, global: 0x6ab488, constant: 0x47db40, fn: 0x4030e0, lib fn: 0x32749547530
    stack: 0x3b5d47d80e4, global: 0x6ab488, constant: 0x47db40, fn: 0x4030e0, lib fn: 0x394469a7530
    stack: 0x3fe2c4e5564, global: 0x6ab488, constant: 0x47db40, fn: 0x4030e0, lib fn: 0x399734a2530
    stack: 0x3e525e0fb24, global: 0x6ab488, constant: 0x47db40, fn: 0x4030e0, lib fn: 0x2f62a810530
    stack: 0x3b50fb3eae4, global: 0x6ab488, constant: 0x47db40, fn: 0x4030e0, lib fn: 0x2e590e86530

After:

    stack: 0x38cf12c90a4, global: 0x3e2d46b488, constant: 0x3e2d23cf80, fn: 0x3e2d1c2510, lib fn: 0x2617d3b4530
    stack: 0x3d733faf474, global: 0x7eb1839488, constant: 0x7eb160af80, fn: 0x7eb1590510, lib fn: 0x32d30c1f530
    stack: 0x3bb42212ec4, global: 0x5bbb365488, constant: 0x5bbb136f80, fn: 0x5bbb0bc510, lib fn: 0x3595e6c1530
    stack: 0x39f678c1ab4, global: 0x22c4e3c488, constant: 0x22c4c0df80, fn: 0x22c4b93510, lib fn: 0x3835b727530
    stack: 0x3afb25bd394, global: 0x493eab2488, constant: 0x493e883f80, fn: 0x493e809510, lib fn: 0x3478d6a7530

This may also be necessary on other platforms, but I can only test on
Linux right now. Note that GDB gained support for debugging position
independent executables in version 7.1 (March 2010).
rustdoc: Set a nonzero exit status on failure

Now that rustdoc is spawning a child task, the program won't exit with a default
error code if the main task fails (because it never fails). This commit forces
the main task to wait for a child task in order to correctly propagate failure.

Closes #16341
Implement Win64 system ABI.
Adding test for issue #15917 which was previously fixed with #15709
libcore: Fix documentation comment for f32.
don't translate items when monomorphizing foreign-ABI functions
Make small doc contributions less onerous/intimidating
mk: Don't emit landing pads in stage 0.

Reduces time to build stage0 from 11:30 to 9:40 on my machine.
auto merge of #16340 : thestinger/rust/pie, r=brson

Rust already builds all code as position independent by default, so the
linker can be told to build a position independent executable if it's
not disabled with `-C relocation-model=dynamic-no-pic`. Position
independent code does have a significant cost on i686 (not on x86_64 or
ARM) but there's no significant cost to linking code that's already
position independent as a position independent executable.

Address space layout randomization makes exploiting vulnerabilities much
more difficult by providing a statistical defence against an attempt to
find or modify existing code / data. Without ASLR, it's trivial to use a
vulnerability to take over control of the process via return-oriented
programming.

Rust code can be used for return-oriented programming whether it is safe
or unsafe, so even a fully safe application needs to be built as a
position independent executable to defend against vulnerabilities in
unsafe blocks or C libraries.

Sample program:

    extern crate libc;

    use std::mem;

    static mut global: u32 = 5;
    static constant: u32 = 5;
    fn foo() {}

    fn main() {
        let local = 5;
        println!("stack: {}, global: {}, constant: {}, fn: {}, lib fn: {}",
                 &local as *const u32,
                 unsafe { &global as *const u32 },
                 &constant as *const u32,
                 unsafe { mem::transmute::<_, *const ()>(foo) },
                 unsafe { mem::transmute::<_, *const ()>(libc::mprotect) });
    }

Before:

    stack: 0x3ff15eb9f94, global: 0x6ab488, constant: 0x47db40, fn: 0x4030e0, lib fn: 0x32749547530
    stack: 0x3b5d47d80e4, global: 0x6ab488, constant: 0x47db40, fn: 0x4030e0, lib fn: 0x394469a7530
    stack: 0x3fe2c4e5564, global: 0x6ab488, constant: 0x47db40, fn: 0x4030e0, lib fn: 0x399734a2530
    stack: 0x3e525e0fb24, global: 0x6ab488, constant: 0x47db40, fn: 0x4030e0, lib fn: 0x2f62a810530
    stack: 0x3b50fb3eae4, global: 0x6ab488, constant: 0x47db40, fn: 0x4030e0, lib fn: 0x2e590e86530

After:

    stack: 0x38cf12c90a4, global: 0x3e2d46b488, constant: 0x3e2d23cf80, fn: 0x3e2d1c2510, lib fn: 0x2617d3b4530
    stack: 0x3d733faf474, global: 0x7eb1839488, constant: 0x7eb160af80, fn: 0x7eb1590510, lib fn: 0x32d30c1f530
    stack: 0x3bb42212ec4, global: 0x5bbb365488, constant: 0x5bbb136f80, fn: 0x5bbb0bc510, lib fn: 0x3595e6c1530
    stack: 0x39f678c1ab4, global: 0x22c4e3c488, constant: 0x22c4c0df80, fn: 0x22c4b93510, lib fn: 0x3835b727530
    stack: 0x3afb25bd394, global: 0x493eab2488, constant: 0x493e883f80, fn: 0x493e809510, lib fn: 0x3478d6a7530

This may also be necessary on other platforms, but I can only test on
Linux right now. Note that GDB gained support for debugging position
independent executables in version 7.1 (March 2010).
librustc: Don't use the same alloca for match binding which we reassign to in arm body.
librustc: Also use new alloca if matching on an arg or upvar which we reassign in the arm body.
librustc: Encode upvar_borrow_map in metadata.
auto merge of #16342 : alexcrichton/rust/issue-16341, r=huonw

Now that rustdoc is spawning a child task, the program won't exit with a default
error code if the main task fails (because it never fails). This commit forces
the main task to wait for a child task in order to correctly propagate failure.

Closes #16341
auto merge of #16346 : vadimcn/rust/win64-cabi, r=brson

This fixes
run-pass/extern-pass-TwoU64s.rs
run-pass/extern-pass-empty.rs
run-pass/extern-return-TwoU64s.rs
auto merge of #16350 : hirschenberger/rust/issue-15917, r=alexcrichton

Adding test for issue #15917 which was previously fixed with #15709
removing 'str'='struct' search synonym

fixes #16384
rustdoc: use raw search in URL

fixes #16385
fixes #16271
auto merge of #16356 : ruud-v-a/rust/fix-f32-doc, r=steveklabnik

On a side note (but maybe this is not the right place to ask), the name `Float::two_pi` is inconsistent with `f32::consts::PI_2`.
auto merge of #16359 : epdtry/rust/mono-item-dedup-foreign, r=alexcrichton

Extend the changes from #16059 to the new generic foreign functions introduced by #15831.
auto merge of #16371 : Gankro/rust/friendly-contrib, r=brson

Had someone experience deep existential dread over building all of Rust on Windows just to do some minor fixes to the docs. This seems entirely unnecessary between reviewers + Bors for most purely english changes.