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

[Fangrui Song via cfe-commits]

https://github.com/MaskRay commented:

Thanks for implementing -mcpu=unset. This looks good. I'm glad to see -mcpu=unset gaining support, especially after feeling like the lone voice in the GCC thread: https://gcc.gnu.org/pipermail/gcc-patches/2025-June/685537.html

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