Keep unstable target features for asm feature checking

Inline assembly uses the target features to determine which registers
are available on the current target. However it needs to be able to
access unstable target features for this.

Fixes rust-lang#99071

src/lib.rs

@@ -167,7 +167,7 @@ impl CodegenBackend for CraneliftCodegenBackend {
  }
  }
 
- fn target_features(&self, _sess: &Session) -> Vec<rustc_span::Symbol> {
+ fn target_features(&self, _sess: &Session, _allow_unstable: bool) -> Vec<rustc_span::Symbol> {
  vec![]
  }