arch: arm: cortex-m: fix PSPLIM configuring in context-switch

When configuring the built-in stack guard, via setting the
PSPLIM register, during thread context-switch, we shall only
set PSPLIM to "guard" the thread's privileged stack area when
the thread is actually using it (PSP is on this stack).

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