Add stage0 guard for cmpxchg16b

Author: alexcrichton

coresimd/x86_64/cmpxchg16b.rs

@@ -42,6 +42,7 @@ use stdsimd_test::assert_instr;
 #[inline]
 #[cfg_attr(test, assert_instr(cmpxchg16b, success = Ordering::SeqCst, failure = Ordering::SeqCst))]
 #[target_feature(enable = "cmpxchg16b")]
+#[cfg(not(stage0))]
 pub unsafe fn cmpxchg16b(
     dst: *mut u128,
     old: u128,