stubs: split semihosting_get_target from system only stubs

As we are about to have a common syscalls.c for gdbstub we need to
stub out one semihosting helper function for all targets.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20230307170405.796822-1-alex.bennee@linaro.org>