RISC-V: selftests: Add which-cpus hwprobe test

Test the RISCV_HWPROBE_WHICH_CPUS flag of hwprobe. The test also
has a command line interface in order to get the cpu list for
arbitrary hwprobe pairs.

Reviewed-by: Palmer Dabbelt <palmer@rivosinc.com>
Signed-off-by: Andrew Jones <ajones@ventanamicro.com>
Link: https://lore.kernel.org/r/20231122164700.127954-10-ajones@ventanamicro.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>