Auto merge of #133417 - RalfJung:aarch64-float-abi, r=workingjubilee

reject aarch64 target feature toggling that would change the float ABI

~~Stacked on top of rust-lang/rust#133099. Only the last two commits are new.~~

The first new commit lays the groundwork for separately controlling whether a feature may be enabled or disabled. The second commit uses that to make it illegal to *disable* the `neon` feature (which is only possible via `-Ctarget-feature`, and so the new check just adds a warning). Enabling the `neon` feature remains allowed on targets that don't disable `neon` or `fp-armv8`, which is all our built-in targets. This way, the entire PR is not a breaking change.

Fixes rust-lang/rust#131058 for hardfloat targets (together with rust-lang/rust#133102 which fixed it for softfloat targets).

Part of rust-lang/rust#116344.

Author: bors