[MC] Make -mcpu=native test target specific (#159868)

It's not possible to use `-mcpu=native` when the Target's Triple doesn't
match the Host's. Move this test to the X86 directory so that it isn't
run while cross-compiling.

Originally #159414

---------

Co-authored-by: Cameron McInally <[email protected]>

Author: mcinally

llvm/test/MC/AsmParser/native.s

@@ -0,0 +1,5 @@
+# REQUIRES: host=x86_64-{{.*}}
+# RUN: llvm-mc -triple=x86_64 -filetype=obj -o %t -mcpu=native %s 2> %t.stderr
+# RUN: FileCheck --allow-empty %s < %t.stderr
+
+# CHECK-NOT: {{.+}}