arch: arm: userspace: z_arm_userspace_enter: reduce push/pop overhead

ARM user space requires ARM_MPU. We can, therefore,
remove the unnecessary #ifdef CONFIG_ARM_MPU blocks
in userspace.S. In addition, we do minor refactoring
in z_arm_userspace_enter(), and z_arm_pendsv(), and
z_arm_svc(), aiming at reducing the push/pop overhead
as much as possible.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>