riscv: hwprobe: export Ztso ISA extension

Export the Ztso extension to userspace.

Signed-off-by: Clément Léger <cleger@rivosinc.com>
Link: https://lore.kernel.org/r/20231220155723.684081-3-cleger@rivosinc.com


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