[llvm] [RISCV] Add groupid/bitmask for RISC-V extension (PR #94440)

[Jessica Clarke via llvm-commits]

jrtc27 wrote:

> I'm not sure we should allocate bits for extensions that aren't supported by hwprobe. Without hwprobe or another enumeration method, the multiversion function that uses a feature that isn't in hwprobe will always never be called. Does that give users false hope?
> 
> Should we try to keep the most common features in the same group as long as we can? Like F and V in the same group.

hwprobe is Linux-specific, this proposed interface (which I don’t love, but at least it’s not hwprobe) is not. And it can still fail at run time if you run on an OS that doesn’t yet support the extension.

https://github.com/llvm/llvm-project/pull/94440