Reland "[AArch64] handle -Wa,-march="

This reverts commit fd11a26d, which was
reverted by 9145a3d4 due to a test
failure on aarch64 backend, e.g.
https://lab.llvm.org/buildbot/#/builders/43/builds/7031. This patch
fixed the test failure.

Reviewed By: DavidSpickett, nickdesaulniers

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