Pre-RFC: Consider statically-enabled target-features when deciding if calling #[target_feature(enable = "foo")] requires unsafe

Summary Currently, calling a function annotated with #[target_feature(enable = "foo")] requires the function making the call also to be annotated with #[target_feature(enable = "foo")] in order for the call not to require unsafe even if "foo" is enabled for the whole compilation. This RFC removes the requirement for unsafe to call a function annotated with #[target_feature(enable = "foo")] when "foo" is enabled for the whole compilation (by any mechanism: --target, -C target-cpu, -C target-featu...