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

[Craig Topper via llvm-commits]

topperc 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.

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