selftests: Test the new RISC-V hwprobe interface

This adds a test for the recently added RISC-V interface for probing
hardware capabilities. It happens to be the first selftest we have for
RISC-V, so I've added some infrastructure for those as well.

Co-developed-by: Palmer Dabbelt <palmer@rivosinc.com>
Signed-off-by: Evan Green <evan@rivosinc.com>
Link: https://lore.kernel.org/r/20230407231103.2622178-6-evan@rivosinc.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>