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

[Camel Coder via llvm-commits]

camel-cdr wrote:

> The ISA is specified in this document too https://www.sifive.com/document-file/eic7700x-datasheet available here https://www.sifive.com/boards/hifive-premier-p550

None of the documentation is consistant nor complete reguarding the ISA:

* [Product Brief](https://www.sifive.com/document-file/hifive-premier-p550-product-brief): Page 2: RV64GBC, Page 3: RV64GC (RV64IMAFDC)
* [HiFive Premier P550 Datasheet](https://www.sifive.com/document-file/hifive-premier-p550-datasheet): RV64GC

I wasn't able to find any other documentation reguarding ISA.

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