[ARM] Clang gives unintended warning message for 'mthumb' + M-profiles:

$ clang --target=armv7m-none-eabi -mthumb -march=armv7-m -c test.c
clang-3.8: warning: argument unused during compilation: '-mthumb'

Differential Revision: http://reviews.llvm.org/D14384

llvm-svn: 252175