arch: arm: restrict IRQ lock to minimum during pendSV exception

When performing thread context-switch it is not necessary to
have IRQs locked while saving the current thread's callee-saved
(and possibly floating point) registers. We only need to lock
the interrupts when accessing the thread ready queue cache.

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