emacs: do not highlight `#foo]` as an attribute

part of #14347
emacs: highlight `#[foo = "bar"]` attributes

Setting `OVERRIDE` to `t` overrides the conflicting highlight Emacs
places on string literals.

Fix #14347
Updated deps
Platform configuration

FIXME: avoid using Xcode build and switch to a patched libuv
Runtime support for arm on iOS
Disable generating split-stack code

Allows to compile for archs which do not have (or have limited)
segmented stack support like embedded.
Basic iOS support
Better dylib skipping based on Alex Crichton code
Relicense shootout-pidigits.rs

Everyone agreed.

Related to #14248, close #14718
Relicense shootout-binarytrees.rs

Everyone agreed.

Related to #14248, close #14720
Cosmetic fixes & comments
There should be no metadata to read on iOS
librustc: Remove useless code to handle `let _ = ...`

This code didn't do anything, but was a vestige of the old semantics for
`let _ = ...`.

Closes #10488. (As near as I can tell anyhow.)
Updated compiler-rt

Previous update unfortunately included changes which
broke Android compilation. This update fixes it and
should allow correct compilation of SjLj for iOS and
all builtins for Android.
Small improvement for json PrettyEncoder
Potential fix for Win32 build

It seems in one of rebases I’ve resolved conflicts wrong and left one redundant line, it is absent in current master and it might cause compilation failure by copying file into itself.
Register new snapshots
auto merge of #14715 : vhbit/rust/ios-pr2, r=alexcrichton
auto merge of #14900 : alexcrichton/rust/snapshots, r=huonw

Closes #14898
Closes #14918
Fix rustdoc's formatting of lifetimes

rustdoc was previously formatting lifetimes with two apostrophes, presumably
as a result of #14797.