Driver: Pass down the -march setting down to -cc1as on x86 too.

The assembler historically didn't make use of any target features, but this has
changed when support for old CPUs that don't support long nops was added.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@175919 91177308-0d34-0410-b5e6-96231b3b80d8