add `naked_functions_target_feature` unstable feature
tracking issue: https://github.com/rust-lang/rust/issues/138568
tagging https://github.com/rust-lang/rust/pull/134213https://github.com/rust-lang/rust/issues/90957
This PR puts `#[target_feature(/* ... */)]` on `#[naked]` functions behind its own feature gate, so that naked functions can be stabilized. It turns out that supporting `target_feature` on naked functions is tricky on some targets, so we're splitting it out to not block stabilization of naked functions themselves. See the tracking issue for more information and workarounds.
Note that at the time of writing, the `target_features` attribute is ignored when generating code for naked functions.
r? ``@Amanieu``
c354a97bd9