rustc: Stop using LLVMGetSectionName

The recent pull request to remove libc from libstd has hit a wall in compiling
on windows, and I've been trying to investigate on the try bots as to why (it
compiles locally just fine). To the best of my knowledge, the LLVM section
iterator is behaving badly when iterating over the sections of the libc DLL.

Upon investigating the LLVMGetSectionName function in LLVM, I discovered that
this function doesn't always return a null-terminated string. It returns the
data pointer of a StringRef instance (LLVM's equivalent of &str essentially),
but it has no method of returning the length of the name of the section.

This commit modifies the section iteration when loading libraries to invoke a
custom LLVMRustGetSectionName which will correctly return both the length and
the data pointer.

I have not yet verified that this will fix landing liblibc, as it will require a
snapshot before doing a full test. Regardless, this is a worrisome situation
regarding the LLVM API, and should likely be fixed anyway.
rustllvm: Remove a no longer needed file

The .def.in files haven't been necessary since the switch to static linking
awhile back.
std: override clone_from for Vec.

A vector can reuse its allocation (and the allocations/resources of any
contained values) when cloning into an already-instantiated vector, so
we might as well do so.
serialize: add a few missing pubs to base64
compiletest: Fix bitrotted win32 routines
test/debug-info: Add/remove ignore-win32 flags

Fixes #10474
test/run-pass: Add/remove some ignore-win32 flags
Removed managed boxes from libarena.
`extern crate` and `mod` are not easily confused

Remove some statements that used to refer to similarities between `mod` and
`extern mod`, before the latter was renamed to `extern crate`.
Fix reference to "these two traits."

There are actually three traits listed.
Minor spelling/grammar/usage fixes.

Note: "different to" is not exactly incorrect, but "different from" is more
commonly accepted in both US and Commonwealth English, and also more
consistent with other usage within this tutorial.
fix Option<~ZeroSizeType>

1778b6361627c5894bf75ffecf427573af02d390 provided the guarantee of no
`exchange_free` calls for ~ZeroSizeType, so a sentinel can now be used
without overhead.

Closes #11998
Update emacs mode to support new `#![inner(attribute)]` syntax.
syntax: Remove AbiSet, use one Abi

This change removes the AbiSet from the AST, converting all usage to have just
one Abi value. The current scheme selects a relevant ABI given a list of ABIs
based on the target architecture and how relevant each ABI is to that
architecture.

Instead of this mildly complicated scheme, only one ABI will be allowed in abi
strings, and pseudo-abis will be created for special cases as necessary. For
example the "system" abi exists for stdcall on win32 and C on win64.

Closes #10049
Test fixes from the rollup
auto merge of #13290 : alexcrichton/rust/rollup, r=alexcrichton

Closes #13285 (rustc: Stop using LLVMGetSectionName)
Closes #13280 (std: override clone_from for Vec.)
Closes #13277 (serialize: add a few missing pubs to base64)
Closes #13275 (Add and remove some ignore-win32 flags)
Closes #13273 (Removed managed boxes from libarena.)
Closes #13270 (Minor copy-editing for the tutorial)
Closes #13267 (fix Option<~ZeroSizeType>)
Closes #13265 (Update emacs mode to support new `#![inner(attribute)]` syntax.)
Closes #13263 (syntax: Remove AbiSet, use one Abi)