arch: arm: userspace: lock swap to set PSP, PSPLIM in userspace enter

When entering user mode, and before the privileged are dropped,
the thread switches back to using its default (user) stack. For
stack limit checking not to lead to a stack overflow, the PSPLIM
and PSP register updates need to be done with PendSV IRQ locked.
This is because context-switch (done in PendSV IRQ) reprograms
the stack pointer limit register based on the current PSP
of the thread. This commit enforces PendSV locking and
unlocking while reprogramming PSP and PSPLIM when switching to
user stack at z_arm_userspace_enter().

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