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
Add Bitwise::{swap_bytes, rotate_left, rotate_right} methods
librustc: Forbid enum-to-float casts.

Closes #14794.

If you're casting from an enum to a float, cast through an integer
first.

[breaking-change]
rustdoc: Larger click areas for sidebar items

* Change links to display:block for click larger targets
* Remove linebreaks due to extra space
* Adjust margins so that element spacing stays the same
* Sidebar item hover background colour chosen from <pre> styling
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.)
getopts: format failure messages with `Show`.

This obsoletes the old `to_err_msg` method. Replace

    println!("Error: {}", failure.to_err_msg())

    let string = failure.to_err_msg();

with

    println!("Error: {}", failure)

    let string = failure.to_str();

[breaking-change]
auto merge of #14866 : bjz/rust/bitwise, r=alexcrichton
getopts: derive Eq for types.
Make analyze_move_out_from use a loop rather than recursion

It will be simpler to make some of the changes that I need to make to
analyze_move_out if it uses a loop rather than recursion.