Make break and continue hygienic

Makes labelled loops hygiene by performing renaming of the labels
defined in e.g. `'x: loop { ... }` and then used in break and continue
statements within loop body so that they act hygienically when used with
macros.

Closes #12262.
auto merge of #12338 : edwardw/rust/hygienic-break-continue, r=cmr

Makes labelled loops hygiene by performing renaming of the labels defined in e.g. `'x: loop { ... }` and then used in break and continue statements within loop body so that they act hygienically when used with macros.
    
Closes #12262.
All uses of `extern fn` should mean `extern "C" fn`. Closes #9309.
auto merge of #12509 : brson/rust/rollup, r=brson

Rollup of https://github.com/mozilla/rust/pull/12506, https://github.com/mozilla/rust/pull/12500, https://github.com/mozilla/rust/pull/12480, https://github.com/mozilla/rust/pull/12478, https://github.com/mozilla/rust/pull/12461.
Update rustc/syntax docs now that rustdoc lexes all non-`notrust` code blocks.

This includes blocks made by indentation, so they need to be changed to
explicitly have ```notrust ... ``` fences..
auto merge of #12510 : huonw/rust/fix-compiler-docs, r=alexcrichton

This includes blocks made by indentation, so they need to be changed to
explicitly have ```notrust ... ``` fences..
auto merge of #12328 : nick29581/rust/abi, r=alexcrichton
std: Make atomics immutable. #11583

In Rust, the strongest guarantee that `&mut` provides is that the memory
pointed to is *not aliased*, whereas `&`'s guarantees are much weaker:
that the value can be aliased, and may be mutated under proper precautions
(interior mutability).

Our atomics though use `&mut` for mutation even while creating multiple
aliases, so this changes them to use 'interior mutability', mutating
through immutable references.