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

[Fangrui Song via cfe-commits]

MaskRay wrote:

> I forgot about tomorrow's LLVM 21 branch so maybe I should merge this based on the support I've received so far?

Looks good!

> https://github.com/toppers what's the status of this in GCC? If it's in there I think we should land/backport this, otherwise after the branch is probably ok.

https://gcc.gnu.org/releases.html GCC has a `X.1` release every year. There should be plenty of time. @kito-cheng 

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