aarch64-softfloat: forbid enabling the neon target feature

Author: RalfJung

compiler/rustc_target/src/target_features.rs

@@ -803,10 +803,10 @@ impl Target {
                 // the use of soft-float, so all we can do here is some crude hacks.
                 match &*self.abi {
                     "softfloat" => {
-                        // This is not fully correct, LLVM actually doesn't let us enforce the softfloat
-                        // ABI properly... see <https://github.com/rust-lang/rust/issues/134375>.
-                        // FIXME: should we forbid "neon" here? But that would be a breaking change.
-                        NOTHING
+                        // LLVM will use float registers when `neon` is available, e.g. for
+                        // calls to built-ins. The only way to ensure a consistent softfloat ABI
+                        // on aarch64 is to never enable `neon`, so we enforce that.
+                        FeatureConstraints { required: &[], incompatible: &["neon"] }
                     }
                     _ => {
                         // Everything else is assumed to use a hardfloat ABI. neon and fp-armv8 must be enabled.