[PATCH] RISC-V: Add semihosting support

[Kito Cheng]

Hi Craig:

The v2 patch is LGTM, verified with qemu again, thank you!

> I can use SEMIHOST_exit_extended, but I will also need to add a check for the
> SH_EXT_EXIT_EXTENDED feature. Can I follow up with this in a later patch?

OK to me :)