also ban soft-float target feature on ARM

Author: RalfJung

compiler/rustc_target/src/target_features.rs

@@ -97,6 +97,7 @@ const ARM_FEATURES: &[(&str, Stability, ImpliedFeatures)] = &[
     ("neon", Unstable(sym::arm_target_feature), &["vfp3"]),
     ("rclass", Unstable(sym::arm_target_feature), &[]),
     ("sha2", Unstable(sym::arm_target_feature), &["neon"]),
+    ("soft-float", Forbidden, &[]), // changes float ABI
     // This is needed for inline assembly, but shouldn't be stabilized as-is
     // since it should be enabled per-function using #[instruction_set], not
     // #[target_feature].