[ARM] Implement -Wa,-mfpu and friends for assemblers
This patch allows Clang to pass on -Wa,-mfpu, -Wa,-mhwdiv and
-Wa,-mcpu to the integrated assembler (via target-features), but
-march is still not being passed, but validated.
In case the command line has both -mxxx and -Wa,-mxxx, we warn
that the naked one will not be used in assembler mode.