syscalls: Export _is_user_context() regardless of CONFIG_USERSPACE
syscalls: Export _is_user_context() regardless of CONFIG_USERSPACE

This commit exports the _is_user_context() function regardless
of the CONFIG_USERSPACE setting. If userspace is enabled, the
value returned depends on the execution context. If userspace
is disabled, the _is_user_context() always returns false.

Signed-off-by: Piotr Zięcik <9ef3595b5f0f4fa8d0ceeb52cc0cf0411a0684dd@nordicsemi.no>

finikorg/zephyr,nashif/zephyr,Vudentz/zephyr,finikorg/zephyr,finikorg/zephyr,nashif/zephyr,galak/zephyr,galak/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,galak/zephyr,Vudentz/zephyr,galak/zephyr,galak/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,Vudentz/zephyr,Vudentz/zephyr,nashif/zephyr