[AArch64] handle -Wa,-march=

This fixed PR#48894 for AArch64. The issue has been fixed for Arm in
https://reviews.llvm.org/D95872

The following rules apply to -Wa,-march with this change:
  - Only compiler options apply to non assembly files
  - Compiler and assembler options apply to assembly files
  - For assembly files, we prefer the assembler option(s) if we have both kinds of option
  - Of the options that apply (or are preferred), the last value wins (it's not additive)

Reviewed By: DavidSpickett, nickdesaulniers

Differential Revision: https://reviews.llvm.org/D103184