Auto merge of #128004 - folkertdev:naked-fn-asm, r=Amanieu

codegen `#[naked]` functions using global asm

tracking issue: rust-lang/rust#90957

Fixes #124375

This implements the approach suggested in the tracking issue: use the existing global assembly infrastructure to emit the body of `#[naked]` functions. The main advantage is that we now have full control over what gets generated, and are no longer dependent on LLVM not sneakily messing with our output (inlining, adding extra instructions, etc).

I discussed this approach with `@Amanieu` and while I think the general direction is correct, there is probably a bunch of stuff that needs to change or move around here. I'll leave some inline comments on things that I'm not sure about.

Combined with rust-lang/rust#127853, if both accepted, I think that resolves all steps from the tracking issue.

r? `@Amanieu`

Author: bors