[ARM] Changes to Arch/CPU handling to help with -Wa,-mfpu etc. (nfc)

To be able to handle -Wa, options in the assembler (ClangAs), we need to
make the handling of options based on the value of the options, not direct
Arguments from the list, since the list is immutable.

No functional change in this patch, but this allows validating of -Wa,-mfpu
and friends in the same way we validate -mfpu and friends, *just* for the
assembler.

llvm-svn: 243352