xtensa: ISS: add argc/argv simcall definitions

There are three simcalls implemented by ISS and QEMU related to
argc/argv processing: SYS_iss_argc (get number of command line
arguments), SYS_iss_argv_size (get size of command line argument block)
and SYS_iss_set_argv (copy command line arguments to virtual guest
address).
Add definitions for these calls to iss/include/platform/simcall.h

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