[clang] [llvm] [RISCV] Relax march string order constraint (PR #78120)

[Fangrui Song via cfe-commits]

https://github.com/MaskRay approved this pull request.

GCC patch has just landed: https://gcc.gnu.org/pipermail/gcc-patches/2024-January/643411.html

https://github.com/llvm/llvm-project/pull/78120