arch: arm: switch to privilege stack in SVC handler

Initialize the privilege stack and switch PSP to it early in the SVC
handler to ensure `z_arm_do_syscall` does not start on a user-accessible
stack frame.

Signed-off-by: Sudan Landge <sudan.landge@arm.com>
(cherry picked from commit 319c69728629d8447e230a0532e22af18eb30dc3)
3 files changed