lit.cfg: Don't expand %ms_abi_triple to non-X86 triples

This is a follow-up to r199260. On ARM hosts, we were attempting to run
tests with triples such as armv7l-unknown-win32. This expands that fix to
cover all non-x86 targets since we only support MS ABI on x86.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@199280 91177308-0d34-0410-b5e6-96231b3b80d8