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_span/src/symbol.rs

@@ -1122,7 +1122,6 @@ symbols! {
         repr_packed,
         repr_simd,
         repr_transparent,
-        reserved_r9: "reserved-r9",
         residual,
         result,
         rhs,