turn ABI-controlling target features into a target modifier

[RalfJung]

This makes it so that we can mark "internal" target features (which we track as target features in the compiler but do not expose as such to the user) as target modifiers, to be really sure that they are set consistently across all targets. For instance, if there was a value for -Ctarget-cpu that affected ABI-relevant target features, we'd catch mismatches in the value of that target feature.

For now, this just complements the ABI check logic in abi_required_features, but I think long-term it might even be able to fully replace that logic. This would, for example, let us check consistency of the SPARC ABI (#160562) without introducing a new variant for RustcAbi. OTOH having the ABI represented explicitly is often useful elsewhere in the compiler (e.g. for inline asm) so we may also want to keep both abi_required_features and this new concept of "target features that must be consistent across all crates". We could also eventually turn the target modifier into a safety net: if abi_required_features violations are a hard error and -Ctarget-feature is locked down to reject non-Rust target features, it should be impossible to ever violate the new target modifier check.

Cc @Darksonn @workingjubilee

• I did not use an LLM to create a change in this PR.
• I used an LLM to create a change in this PR, and I have explained below how it was used.

Stacked on top of #160530, only the last commit is new.

[rustbot]

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[RalfJung]

FWIW I could not quite understand why we have both this map and some extra logic in gather_target_modifiers. When I found this map I had assumed that the entire contents of this map will be stored as target modifier, but instead there's a separate filtering step. Why?

Cc @azhogin

View changes since the review

[RalfJung]

r? @davidtwco