Turn stdarch into a Josh subtree

[Kobzol]

In a similar vein as #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 1b4d15df12079504942d0a3f1030b2039b8a776c, which is the previous commit SHA of the stdarch submodule. The sync was performed according to https://hackmd.io/7pOuxnkdQDaL1Y1FQr65xg.

This was decided in rust-lang/stdarch#1655.

Test pull PR on my fork: Kobzol/stdarch#1
Test push PR on my fork: Kobzol#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

[RalfJung]

For Miri, we basically never change the lockfile in the rustc repo. I do manual cargo updates in the Miri repo every now and then.

[Kobzol]

Oh, nevermind, stdarch actually doesn't have a lockfile, that's what was causing the issue. Sorry! The same issue actually happens also for https://github.com/rust-lang/compiler-builtins, as it also misses a lockfile, and it also causes bootstrap problems.

[Kobzol]

@bors2 try

[rust-bors]

⌛ Trying commit 11d63c5 with merge 594fc6f…

To cancel the try build, run the command @bors2 try cancel.

[rust-bors]

💔 Test failed

• CI ❌

[Kobzol]

This will need a stdarch bump with the avx512 features removed.

[bors]

☔ The latest upstream changes (presumably #142443) made this pull request unmergeable. Please resolve the merge conflicts.

[Kobzol]

Rebased the PR on top of latest master, which included a stdarch update that now makes the bootstrap smoke test work. This should be ready now.

[rustbot]

⚠️ Warning ⚠️

• There are issue links (such as #123) in the commit messages of the following commits.
  Please move them to the PR description, to avoid spamming the issues with references to the commit, and so this bot can automatically canonicalize them to avoid issues with subtree.

  • ab098c6
  • 6e4ad9c
  • e61df09
  • 68c0308

• The following commits have merge commits (commits with multiple parents) in your changes. We have a no merge policy so these commits will need to be removed for this pull request to be merged.

  • f59d072

  You can start a rebase with the following commands:

  $ # rebase
  $ git pull --rebase https://github.com/rust-lang/rust.git master
  $ git push --force-with-lease
  

[Kobzol]

@bors2 try

[rust-bors]

⌛ Trying commit d97a99a with merge f871e6e…

To cancel the try build, run the command @bors2 try cancel.

[RalfJung]

Did we have successful compiler-builtin syncs in both directions, to confirm that the basic approach works?