selftests: Test RISC-V Vector prctl interface

This add a test for prctl interface that controls the use of userspace
Vector.

Signed-off-by: Andy Chiu <andy.chiu@sifive.com>
Link: https://lore.kernel.org/r/20230605110724.21391-27-andy.chiu@sifive.com


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