RISC-V: Support pointer masking extension 1.0

- Adding Ssnpm, Smnpm, Smmpm, Sspm, and Supm
- No new CSR added
- Pointer masking only applies to RV64
- Ref: https://github.com/riscv/riscv-j-extension/releases/download/pointer-masking-ratified/pointer-masking-ratified.pdf

Signed-off-by: Jerry Zhang Jian <jerry.zhangjian@sifive.com>