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

[Craig Topper via llvm-commits]

topperc 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.

Page 69 of https://www.sifive.com/document-file/eic7700x-datasheet seems to be the best reference. I'll try to get the other documents fixed.

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