add option to filter away unsupported target features

[RalfJung]

See #5294 for discussion: with this option, aarch64 code that checks cfg!(target_feature = "aes") and has a fallback path will automatically do the right thing in Miri.

Unfortunately we cannot use a -Zmiri flag here as only the actual binary sees that flag, but all crates need to to have their cfg updated. So I made it a new environment variable instead. Kind of hacky but I cannot think of anything better...

[RalfJung]

@rust-lang/miri This is a first step in the context of #5294, without committing to any kind of transition of defaults: one can set MIRI_DISABLE_UNSUPPORTED_TARGET_FEATURES=1 to have Miri remove unsupported target features from cfg(target_feature) to steer code towards its fallback paths. Unfortunately, we cannot use a -Zmiri flag here as only the actual binary sees that flag, but all crates need to to have their cfg updated. So I made it a new environment variable instead. If you have a better idea for doing this, please let me know.

[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.