Rollup merge of #148253 - bjorn3:dummy_backend_target_features, r=JonathanBrouwer

Handle default features and -Ctarget-features in the dummy backend

This prevents a warning about ABI relevant target features not being set on x86 and arm. In addition it is required for miri to report correct features in is_*_feature_detected!() if miri switches to the dummy backend.

Required for rust-lang/miri#4648

Author: jhpratt