[llvm] [clang] [RISCV] Add support for RISC-V Pointer Masking (PR #79929)

[Jesse Huang via cfe-commits]

https://github.com/jaidTw commented:

Should you also update the `riscv32-toolchain-extra.c` and `riscv64-toolchain-extra.c`?
LGTM otherwise

https://github.com/llvm/llvm-project/pull/79929