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

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.

Reviewed By: asb

Differential Revision: https://reviews.llvm.org/D146451