arch: arm: cortex-m: move PSPLIM clearing in the relevant function

We do not need to have the PSPLIM clearing directly inside
the PendSV handler and outside the function that configures
it, configure_builtin_stack_guard(), since the latter is also
invoked inside the PendSV handler. This commit moves the
PSPLIM clearing inside configure_builtin_stack_guard(). The
patch is not introducing any behavioral change on the
stack limit checking mechanism for Cortex-M.

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