arch: arm: userspace: don't use the default stack in z_arm_do_syscall

z_arm_do_syscall is executing in privileged mode. This implies
that we shall not be allowed to use the thread's default
unprivileged stack, (i.e push to or pop from it), to avoid any
possible stack corruptions.

Note that since we execute in PRIV mode and no MPU guard or
PSPLIM register is guarding the end of the default stack, we
won't be able to detect any stack overflows.

This commit implement the above change, by forcing
z_arm_do_syscall() to FIRST switch to privileged
stack and then do all the preparations to execute
the system call.

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