riscv: Add hwprobe vdso call support

The new riscv_hwprobe syscall also comes with a vDSO for faster answers
to your most common questions. Call in today to speak with a kernel
representative near you!

Signed-off-by: Evan Green <evan@rivosinc.com>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>