Slight copy-editing for `std::cell::Cell` docs

Hopefully this is a bit more precise and also more correct English.

Change the ICE from #55223 to a hard error

cc @SimonSapin

r? @RalfJung

fixes https://github.com/rust-lang/rust/issues/55287

[CI] Run a `thumbv7m-none-eabi` binary using `qemu-system-arm` [IRR-2018-embedded]

## What's included?

- Run a `thumbv7m-none-eabi` binary using `qemu-system-arm`
- We are using `cortex-m-rt = "=0.5.4"` which does not use `proc_macro`.
(reason: stage2 build of rustc does not work well with `proc_macro` in `run-make` phase.)
- We are using GNU LD for now.

## Blocker

All resolved.
- ~[Waiting] `#[panic_handler]` is not available in stable.~
  - [Merged] https://github.com/rust-lang/rust/pull/53619
- ~[Waiting] https://github.com/japaric/lm3s6965evb: does not compile on stable.~
  - [OK] dependent crate ~`panic-abort`~ `panic-halt`: already moved to use `#[panic_handler]`.

## Update

`#[panic_handler]` will be stabilized in Rust 1.30.

CC @kennytm @jamesmunns @nerdyvaishali

[breaking change] update stdsimd

r? @alexcrichton

Rollup of 11 pull requests

Successful merges:

 - #55490 (resolve: Fix ICE in macro import error recovery)
 - #55597 (std: Enable usage of `thread_local!` through imports)
 - #55601 (Fix tracking issue numbers for some unstable features)
 - #55621 (Add precision for create_dir function)
 - #55644 (ci: Add Dockerfile for dist-powerpcspe-linux)
 - #55664 (Make "all possible cases" help message uniform with existing help messages)
 - #55689 (miri: binary_op_val -> binary_op_imm)
 - #55694 (Fixes #31076)
 - #55696 (NLL Diagnostic Review 3: Missing errors for borrows of union fields)
 - #55700 (Update ui tests with respect to NLL)
 - #55703 (Update `configure --help` (via configure.py) to reflect decoupling of debug+optimize)

Update `configure --help` (via configure.py) to reflect decoupling of debug+optimize

This should have been part of PR #54811 (my bad).

Update ui tests with respect to NLL

Fix #55533

NLL Diagnostic Review 3: Missing errors for borrows of union fields

Fixes #55675.

This PR modifies a test to make it more robust (it also fixes indentation on a doc comment, but that's not the point of the PR). See the linked issue for details.

r? @pnkfelix

Fixes #31076

miri: binary_op_val -> binary_op_imm

Seems like this was overlooked during the value -> immediate rename.

r? @oli-obk

Make "all possible cases" help message uniform with existing help messages

Specifically no capitalisation or trailing full stops.

ci: Add Dockerfile for dist-powerpcspe-linux

This adds the Dockerfile for cross-building Rust for the powerpcspe target. It's currently disabled.

Add precision for create_dir function

Took me a while to find the other equivalent so it seems to be necessary.

r? @QuietMisdreavus

Fix tracking issue numbers for some unstable features

And also remove deprecated unstable `#[panic_implementation]` attribute that was superseded by stable `#[panic_handler]` and doesn't have an open tracking issue.

std: Enable usage of `thread_local!` through imports

The `thread_local!` macro delegated to an internal macro but it didn't
do so in a macros-and-the-module-system compatible fashion, meaning if a
`#![no_std]` crate imported `std` and tried to use `thread_local!` it
would fail due to missing a lookup of an internal macro.

This commit switches the macro to instead use `$crate` to invoke other
macros, ensuring that it'll work when `thread_local!` is imported alone.

resolve: Fix ICE in macro import error recovery

Fixes https://github.com/rust-lang/rust/issues/55457

std: Improve codegen size of accessing TLS

Some code in the TLS implementation in libstd stores `Some(val)` into an
`&mut Option<T>` (effectively) and then pulls out `&T`, but it currently
uses `.unwrap()` which can codegen into a panic even though it can never
panic. With sufficient optimizations enabled (like LTO) the compiler can
see through this but this commit helps it along in normal mode
(`--release` with Cargo by default) to avoid codegen'ing the panic path.

This ends up improving the optimized codegen on wasm by ensuring that a
call to panic pulling in more file size doesn't stick around.

Use lld directly for Fuchsia target

Fuchsia already uses lld as the default linker, so there's no reason
to always invoke it through Clang, instead we can simply invoke lld
directly and pass the set of flags that matches Clang.

Fuchsia already uses lld as the default linker, so there's no reason
to always invoke it through Clang, instead we can simply invoke lld
directly and pass the set of flags that matches Clang.

Fix undefined behavior in Rc/Arc allocation

Manually calculate allocation layout for `Rc`/`Arc` to avoid undefined behavior

Closes #54908

Correct alignment of atomic types and (re)add Atomic{I,U}128

This is a updated https://github.com/rust-lang/rust/pull/53514 to also make atomic types `repr(C)` as per comment in https://github.com/rust-lang/rust/pull/53514#issuecomment-431042767.

Fixes #39590
Closes #53514

r? @pnkfelix

Manually calculate allocation layout for `Rc`/`Arc` to avoid undefined behavior

Custom diagnostic when trying to doc comment argument

When writing

```
pub fn f(
    /// Comment
    id: u8,
) {}
```

Produce a targeted diagnostic

```
error: documentation comments cannot be applied to method arguments
  --> $DIR/fn-arg-doc-comment.rs:2:5
   |
LL |     /// Comment
   |     ^^^^^^^^^^^ doc comments are not allowed here
```

Fix #54801.

add call to tcx.sess.delay_span_bug before returning none to make sure error is presented to user

NLL has increased precision in its analysis of drop order, and we want
the test annotations to deliberately reflect this by having fewer
ERROR annotations for NLL than for AST-borrowck. The best way to get
this effect is via `// revisions`.

As a drive-by, also added uses of all the borrows just to make it
clear that NLL isn't somehow sidestepping things by using shorter
borrows than you might have otherwise expected. (Of course, the added
uses do not make all that much difference since the relevant types all
declare `impl Drop` and thus those drops have implicit uses anyway.)

This is a variant of `ui/borrowck/borrowck-closures-mut-of-imm.rs`
that I used to help identify what changes I needed to make to the
latter file in order to recover its instances of E0524 under NLL.

(Basically this test includes the changes you'd need to make to
`ui/borrowck/borrowck-closures-mut-of-imm.rs` in order to get rid of
occurrences of E0596. And then I realized that one needs to add
invocations of the closures in order to properly extend the mutable
reborrows in a manner such that NLL will roughly match AST-borrowck.)

This is based on the feedback from estebank:

"""
I believe that test can be removed outright. It'd be impossible for a
new change to go through that breaks this kind of output without it
being picked up by multiple other `stderr` tests. This is an artifact
of the transition period to the "new" output style.
"""

see: https://github.com/rust-lang/rust/issues/52663#issuecomment-422155551

This is not strictly necessary to make this test "more robust with
respect to NLL"; its just an attempt to narrow the scope of the test
and focus on its core.