[llvm] [Exegesis][RISCV] Add initial RVV support (PR #128767)

[Nico Weber via llvm-commits]

nico wrote:

It's unusual to include a FooSubtarget.h header outside of lib/Target (the subtarget header includes a bunch of llvm-tblgen'd files, so this serializes the build, etc). Is there a way to prevent this inclusion?

https://github.com/llvm/llvm-project/pull/128767