Adapt Rust's LLVM wrapper to CHERIoT-Platform's LLVM

[xdoardo]

This is mostly a git am of this commit on top of our beta branch, with minor changes. Depends on this PR being merged first.

Note that since now (cf. 1.72) memmove and memcpy are part of the BuilderMethods trait, the PreserveCheriTags parameters have leaked into GCC as well. Curiously, Cranelift does not complain about it as there are no implementations of BuilderMethods in rustc_codegen_cranelift.

About CI

This is also a test of the CI set up in the other PR. I think it doesn't make a lot of sense to have it fail at the "check commit" step: perhaps this check could be skipped if the branch the PR comes from is not at distance >1 from the target branch?

[seharris]

Looks ok to me.

By way of a minor nitpick, there are a few files where I feel like it might be slightly tidier to import rustc_codegen_ssa::common::PreserveCheriTags, unless there's a reason not to I haven't noticed? (this wouldn't work in compiler/rustc_codegen_llvm/src/builder.rs because it would conflict with another enum)

I'm thinking of these, though the ones with (?) only use the enum once:

• compiler/rustc_codegen_gcc/src/builder.rs
• compiler/rustc_codegen_gcc/src/intrinsic/mod.rs (?)
• compiler/rustc_codegen_llvm/src/va_arg.rs (?)
• compiler/rustc_codegen_ssa/src/mir/block.rs (?)

[xdoardo]

1ef0eba patches those files as suggested.