[clang] [llvm] [RISCV] Add generic CPUs for profiles (PR #84877)

[Alex Bradbury via cfe-commits]

asb wrote:

We discussed this on the sync-up call and @preames very rightly pointed out that we should take a step back here...from a user perspective, what does specifying a profile via `-mcpu` provide that specifying it via `-march` doesn't? We weren't able to answer that in the call, but perhaps we're missing something?

https://github.com/llvm/llvm-project/pull/84877