[clang] [RISCV] Add -march=unset to cancel and ignore a previous -march. (PR #148321)

[Kito Cheng via cfe-commits]

kito-cheng wrote:

GCC has landed `-march=unset` now :) https://github.com/gcc-mirror/gcc/commit/aa1ae5284da3377190c7dd1e0dbd865bb41c46e6 

https://github.com/llvm/llvm-project/pull/148321