fix: running the test only on x86_64.

The test was failing on aarch64-apple-darwin.

tests/ui/simd/empty-simd-vector-in-operand.rs

@@ -1,4 +1,5 @@
 // Regression test for issue #134224.
+//@ only-x86_64
 
 #![feature(repr_simd)]
 

tests/ui/simd/empty-simd-vector-in-operand.stderr

@@ -1,11 +1,11 @@
 error[E0075]: SIMD vector cannot be empty
-  --> $DIR/empty-simd-vector-in-operand.rs:6:1
+  --> $DIR/empty-simd-vector-in-operand.rs:7:1
    |
 LL | struct A();
    | ^^^^^^^^
 
 error: use of empty SIMD vector `A`
-  --> $DIR/empty-simd-vector-in-operand.rs:11:43
+  --> $DIR/empty-simd-vector-in-operand.rs:12:43
    |
 LL |         std::arch::asm!("{}", in(xmm_reg) A());
    |                                           ^^^