Add a notion of "some ABIs require certain target features"

[RalfJung]

I think I finally found the right shape for the data and checks that I recently added in #133099, #133417, #134337: we have a notion of "this ABI requires the following list of target features, and it is incompatible with the following list of target features". This removes all the "toggleability" stuff introduced before, though we do keep the notion of a fully "forbidden" target feature -- this is needed to deal with target features that are actual ABI switches, and hence are needed to even compute the list of required target features.

This is best reviewed commit-by-commit. The first commit adds the new abi_required_features function. For now, that function supports x86, ARM, and RISC-V (just like the previous logic did). Unsurprisingly, RISC-V is the nicest. ;) Both -Ctarget-feature and #[target_feature] are updated to ensure we follow the rules of the ABI. We also always explicitly (un)set all required and in-conflict features, just to avoid potential trouble caused by the default features of whatever the base CPU is.

As a side-effect this also (unstably) allows enabling x87 when that is harmless.

This should also prepare us for requiring SSE on x86-32bit when we want to use that for our ABI (and for float semantics sanity), see #133611.

The last commit marks SSE2 as required on x86-64, to better match the actual logic in LLVM and because all x86-64 chips do have SSE2.

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These commits modify compiler targets.
(See the Target Tier Policy.)

Some changes occurred in compiler/rustc_codegen_gcc

cc @antoyo, @GuillaumeGomez

[workingjubilee]

I will get around to this, I just put a freeze on my queue because I was accumulating enough reviews in addition to ongoing patch series. :P

[taiki-e]

Suggested change

	"ilp32" | "lp64" => NOTHING,
	"ilp32" | "lp64" => {
	// Incompatible with e.
	(&[], &["e"])
	}

[RalfJung]

Ah right the old logic treated all the non-e ABIs the same, we should keep that.

The old logic also allowed disabling e when using an e ABI, so we should probably keep that, too.

[RalfJung]

@ojeda wrote

(currently, arm64 in the kernel uses --target=aarch64-unknown-none -Ctarget-feature="-neon").

That no longer works after this PR (neon is a required ABI feature so it cannot be disabled with -Ctarget-feature)... so I probably need to at least slightly adjust my strategy here.