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 <piotr.ziecik@nordicsemi.no>