Tracking issue for all the ways in which -C compiler flags can alter the ABI

If a -C flag alters the ABI, mixing crates built with different flags causes UB. This issue is gathering all the ways in which this can happen, so that we can figure out what to do with them. The general goal is to make it impossible to alter these flags without realizing that they are "special" and need to be set consistently across all crates. Ideally rustc can even check that they are set consistently, though that will not cover dynamic linking.

• -Ctarget-features can affect the ABI in a bunch of ways
  • https://github.com/rust-lang/rust/issues/116558
  • https://github.com/rust-lang/rust/issues/116344
  • https://github.com/rust-lang/rust/issues/131058
  • https://github.com/rust-lang/rust/issues/131819
  • https://github.com/rust-lang/rust/issues/64609
  • https://github.com/rust-lang/rust/issues/89586
• https://github.com/rust-lang/rust/issues/131300
  • might also include -Ccode-model
• https://github.com/rust-lang/rust/issues/129893
• -Cllvm-args can set all sorts of flags, some of which can change the ABI

Flags that are sus:

• -Cno-redzone could potentially be a problem
• Probably -Clink-arg(s) can also do bad shenanigans... but it can't really affect ABI, can it? The ABI is already baked into the object files at this point.

This list might be incomplete!

Start by reading the linked issues on -Ctarget-features, -Cllvm-args, and the other suspected flags, then review the referenced compiler discussion. Done means identifying the ABI-affecting flags and agreeing how rustc should ensure they are configured consistently, including the stated dynamic-linking limitation.