[clang] [llvm] [RISCV] Add NutShell RV32/64 processors definition (PR #102899)

[Jessica Clarke via cfe-commits]

jrtc27 wrote:

Some relevant context for what CPUs we permit merging into LLVM: https://github.com/llvm/llvm-project/pull/102452#issuecomment-2275962014

How does this apply to NutShell?

https://github.com/llvm/llvm-project/pull/102899