On ARM, use relocation_model to detect whether r9 should be reserved

The previous approach of checking for the reserve-r9 target feature
didn't actually work because LLVM only sets this feature very late when
initializing the per-function subtarget.

compiler/rustc_codegen_ssa/src/target_features.rs

@@ -36,7 +36,6 @@ 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>)] = &[