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

[Mehdi Amini via cfe-commits]

joker-eph wrote:

Sorry, had to revert because this broke a buildbot:

```
llvm/lib/Support/RISCVISAInfo.cpp:910:18: error: call to deleted constructor of 'llvm::Error'
return E;
^
```
https://lab.llvm.org/buildbot/#/builders/61/builds/53704

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