arch: arm: userspace: lock swap to set PSP & PSPLIM in syscall return

When returning from a system call, the thread switches back
to using its default (user) stack. For stack limit checking
not to lead to a stack overflow, the updates of PSPLIM and
PSP registers 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 returning
from a system call.

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