[llvm] [clang] Revert "[RISCV] Relax march string order constraint" (PR #79976)

[David Blaikie via cfe-commits]

dwblaikie wrote:

(Please don't send pull requests that aren't going to be reviewed - commit directly instead. Otherwise it's unclear which pull requests are expecting review (& should wait on it) and which pull requests don't need review. Especially for new contributors, it'd be good to not give the impression that pull requests should be sent, then committed if reviews are not forthcoming)

https://github.com/llvm/llvm-project/pull/79976