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

[Pengcheng Wang via llvm-commits]

wangpc-pp wrote:

> > I thought the P550 was supposed to support RV64GCB, but that would require Zbs support, which isn't listed here.
> 
> The P550 version in the HiFive Premier board does not support Zbs. ISA string from /proc/cpuinfo is `rv64imafdch_zicsr_zifencei_zba_zbb_sscofpmf`

That's weird. Basically, remving `Zbs` won't save any area. Is it because the kernel is using wrong device tree without Zbs?

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