Use i686 as the ISA in %microsoft_abi_triple on ARM

Long term we should make -triple arm7l-*-win32 do something sensible,
but for now it's broken and untested.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@199260 91177308-0d34-0410-b5e6-96231b3b80d8