[ExpandMemCmp] Add lit.local.cfg

To prevent AArch64 tests from running when the target is not compiled.

Fixes r371502:

/home/buildslave/ps4-buildslave4/llvm-clang-lld-x86_64-scei-ps4-ubuntu-fast/llvm.src/test/Transforms/ExpandMemCmp/AArch64/memcmp.ll:11:15: error: CHECK-NEXT: expected string not found in input
; CHECK-NEXT: [[TMP0:%.*]] = bitcast i8* [[S1:%.*]] to i64*

llvm-svn: 371503