Auto merge of #141899 - Kobzol:stdarch-josh, r=Amanieu

Turn `stdarch` into a Josh subtree

In a similar vein as https://github.com/rust-lang/rust/pull/141229, this PR makes the `stdarch` repository a Josh subtree (it was previously a submodule). The initial commit of `stdarch` upon this is based is `5a7342fc16b208b1b16624e886937ed8509a6506`, which is the previous commit SHA of the `stdarch` submodule. The sync was performed according to https://hackmd.io/7pOuxnkdQDaL1Y1FQr65xg.

This was decided in https://github.com/rust-lang/stdarch/issues/1655.

Test pull PR on my fork: https://github.com/Kobzol/stdarch/pull/1
Test push PR on my fork: https://github.com/Kobzol/rust/pull/59

I plan to use the same Rust (miri-inspired) tooling that we use for `rustc-dev-guide` to enable pulls/pushes on stdarch.

Note that this repository currently doesn't have any stdarch-specific tests, so before that, the subtree should only be modified through this repository only when dealing with changes that contain "cyclical dependencies" between stdarch and rustc. The long term vision is to integrate stdarch into rust-lang/rust completely.

CC `@Amanieu`

try-job: aarch64-apple
try-job: aarch64-gnu
try-job: `x86_64-msvc-*`
try-job: x86_64-gnu
try-job: x86_64-gnu-aux