Rename extern "unadjusted" to extern "llvm-intrinsic"

[bjorn3]

This makes it clear that it is only meant for LLVM intrinsics and not for defining or calling arbitrary user functions.

Follow up to #160077

[rustbot]

compiler-builtins is developed in its own repository. If possible, consider making this change to rust-lang/compiler-builtins instead.

cc @tgross35

This PR changes rustc_public

cc @oli-obk, @celinval, @ouz-a, @makai410

miri is developed in its own repository. If the Miri part of this change can be broken out, consider making this change to rust-lang/miri instead. However, if Miri needs adjusting for rustc changes, just ignore this message.

cc @rust-lang/miri

stdarch is developed in its own repository. If possible, consider making this change to rust-lang/stdarch instead.

cc @Amanieu, @folkertdev, @sayantn

[rustbot]

r? @tgross35

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[RalfJung]

I love it. :)
I only carefully looked at the Miri parts, those LGTM.

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[bjorn3]

CI passed. @tgross35 would you be able to take a look soon. This PR is fairly large, so it may be a bit bitrot prone.