stdarch subtree update Subtree update of `stdarch` to https://github.com/rust-lang/stdarch/commit/50134e10cbd2f81a068259d7796aff2471aae7c1. Created using https://github.com/rust-lang/josh-sync. The only interesting commit is the final one, which enables the `avx10_target_feature` feature in the standard library, because it is now used in `stdarch`. r? `@sayantn` (or whoever, this is just a straightforward sync)