xtensa: ISS: add GDBIO implementation to semihosting interface

Add GDBIO implementation for the xtensa semihosting interface. It offers
less functions than the simcall interface, so make some semihosting
functions optional and return error when implementation is not
available.
Add Kconfig menu to select semihosting implementation and add simcall and
GDBIO choices there.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>