[llvm-objdump] Require x86 target for mcpu/attr test

This fixes test failure on clang-cmake-armv7-quick bot
with change c2ead57.

This bot only builds Arm/AArch64 targets.

Author: DavidSpickett

llvm/test/tools/llvm-objdump/mattr-mcpu-help.test

@@ -1,6 +1,7 @@
 # RUN: yaml2obj %s -o %t
 # RUN: llvm-objdump -d %t --mattr=help 2>&1 | FileCheck %s
 # RUN: llvm-objdump -d %t --mcpu=help 2>&1 | FileCheck %s
+# REQUIRES: x86-registered-target
 
 # CHECK: Available CPUs for this target:
 # CHECK: Available features for this target: