arch: arm: userspace: set PSPLIM to guard default stack in SVCall

Thread will be in privileged mode after returning from SCVall. It
will use the default (user) stack before switching to the privileged
stack to execute the system call. We need to protect the user stack
against stack overflows until this stack transition. We update the
note in z_arm_do_syscall(), stating clearly that it executing with
stack protection when building with stack limit checking support
(ARMv8-M only).

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