accel: Implement accel_init_ops_interfaces() for both system/user mode

accel: Implement accel_init_ops_interfaces() for both system/user mode

We want to build more common code, moving objects from meson's
specific_ss[] set to common_ss[]. Since the CONFIG_USER_ONLY
definitions isn't applied on the common_ss[] set, it is simpler
to add an empty accel_init_ops_interfaces() stub on user emulation,
removing any CONFIG_USER_ONLY use in accel-target.c.

Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20250417165430.58213-5-philmd@linaro.org>