asm: Allow using r9 (ARM) and x18 (AArch64) if they are not reserved by

the current target.

compiler/rustc_codegen_ssa/src/target_features.rs

@@ -36,6 +36,7 @@ const ARM_ALLOWED_FEATURES: &[(&str, Option<Symbol>)] = &[
     // #[target_feature].
     ("thumb-mode", Some(sym::arm_target_feature)),
     ("thumb2", Some(sym::arm_target_feature)),
+    ("reserve-r9", Some(sym::arm_target_feature)),
 ];
 
 const AARCH64_ALLOWED_FEATURES: &[(&str, Option<Symbol>)] = &[