[RISCV] Replace RISCV -> RISC-V in comments. NFC

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.

More patches will follow.

Reviewed By: asb

Differential Revision: https://reviews.llvm.org/D146449