[RISCV] Replace RISCV->RISC-V in strings.

[craig.topper]

Summary

To be consistent with RISC-V branding guidelines
https://riscv.org/about/risc-v-branding-guidelines/
Think we should be using RISC-V where possible.

D146449 already updated comments. Strings may have more user impact.

[asb]

The logic I saw with the pass names is that the backend for the RISC-V architecture is called "RISCV". But I've got no objection to changing.

[craig.topper]

In D146451#4207261, @asb wrote:

The logic I saw with the pass names is that the backend for the RISC-V architecture is called "RISCV". But I've got no objection to changing.

Yeah. I also wasn't sure which is best.

[asb]

Not as compelling as user facing strings or comments that clearly refer to the RISC-V ISA rather than the "RISCV" LLVM backend. But I don't really see a good reason not to be consistent. LGTM.