[clang] [clang][AArch64] multilib: fix deduction of "-march=" option (PR #81474)

[Dominik Wójt via cfe-commits]

domin144 wrote:

@mplatings Can you merge for me? I don't have needed rights.
I rebased hoping the check will pass now, but there are still some unrelated failures on windows.

https://github.com/llvm/llvm-project/pull/81474