[ARM] Error out on .arm assembler directives on windows

Windows on arm is thumb only.

Differential Revision: https://reviews.llvm.org/D43005

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@328298 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed