[MC] Make -mcpu=native test target specific

It's not possible to use `-mcpu=native` when the Target's Triple doesn't match
the Host's. Move this test to the AArch64 directory so that it isn't run while
cross-compiling.

Originally llvm#159414

Author: Cameron McInally

llvm/test/MC/AArch64/mcpu-native.s

@@ -0,0 +1,5 @@
+# REQUIRES: aarch64-registered-target,system-linux,aarch64-host
+# RUN: llvm-mc -triple=aarch64 -filetype=obj -o %t -mcpu=native %s 2> %t.stderr
+# RUN: FileCheck --allow-empty %s < %t.stderr
+
+# CHECK-NOT: {{.+}}