[AArch64] Add missing field 'GuardedControlStack' initializer (NFC)

llvm-project/llvm/lib/TargetParser/ARMTargetParserCommon.cpp:143:39:
 error: missing field 'GuardedControlStack' initializer [-Werror,-Wmissing-field-initializers]
  PBP = {"none", "a_key", false, false};
                                      ^
1 error generated.