[clang] [llvm] [RISCV] Add -mcpu=sifive-p550. (PR #122164)

[Camel Coder via llvm-commits]

camel-cdr wrote:

I thought the P550 was supposed to support RV64GCB, but that wouls require Zbs support, which isn't listed here.

https://github.com/llvm/llvm-project/pull/122164