getopts: cleanup, renames, remove reexports

* Remove public reexports, as a part of #19253
* Rename getopts::Fail_ to getopts::Fail
 * Didn't see a reason for the suffixed '_'
* Removed getopts::FailType
 * Looked like it was only beings used for tests; refactored the tests
   to stop requiring it
* A few other non-breaking trivial refactoring changes

[breaking-change]
librustrt: stack_overflow support mipsel linux
impl Str for CowString

This implementation existed on MaybeOwned, but has been lost in the
transition to Cows. Let's put it back.
auto merge of #19366 : liigo/rust/mipsel-linux, r=alexcrichton

Since #19076 was merged, I believe mipsel + linux maybe need add to the list here, too.
reword faq to remove reference to indexing strings

Fixes #19344
auto merge of #19345 : steveklabnik/rust/gh19344, r=alexcrichton

Fixes #19344
Fix a simple typo
Fix rustc panic on second compile_input
auto merge of #19401 : MatejLach/rust/guide_are_fix, r=steveklabnik

Fixes a small omission of `are` in the sentence:
`There also a few things you can do with a tuple as a whole, without...`
r @steveklabnik?
Replace some verbose match statements with their `if let` equivalent.

No semantic changes, no enabling `if let` where it wasn't already enabled.
auto merge of #19392 : murarth/rust/rustc-compile-twice, r=nick29581

Closes #19371
syntax: Make `asm!` clobbers a proper vector.

Otherwise `--pretty expanded` diverges.
auto merge of #19365 : frewsxcv/rust/getopts-cleanup, r=alexcrichton

* Remove public reexports, as a part of #19253
* Rename getopts::Fail_ to getopts::Fail
 * Didn't see a reason for the suffixed '_'
* Removed getopts::FailType
 * Looked like it was only beings used for tests; refactored the tests
   to stop requiring it
* A few other non-breaking trivial refactoring changes

[breaking-change]
Adjust some error messages to start with a lowercase letter and not finish with a full stop
mk: Use host llvm linkage paths, not target ones

We only build LLVM for the host architecture, not the target architecture, so
this was just a minor typo in the parameters uses.

Closes #19383
Fix the ordering of `unsafe` and `extern` in methods

This breaks code that looks like this:

    trait Foo {
        extern "C" unsafe fn foo();
    }

    impl Foo for Bar {
        extern "C" unsafe fn foo() { ... }
    }

Change such code to look like this:

    trait Foo {
        unsafe extern "C" fn foo();
    }

    impl Foo for Bar {
        unsafe extern "C" fn foo() { ... }
    }

Fixes #19398.

[breaking-change]
trans: Eliminated redundant allocations.
auto merge of #19369 : seanmonstar/rust/cow-str, r=alexcrichton

This implementation existed on MaybeOwned, but has been lost in the
transition to Cows. Let's put it back.

@aturon r?
Fix typo in tests makefile
allow macro expansions in attributes