arm: handle time slicing before context switch on exception.

Currently Thread time slice is getting reset at end of timer
interrupt. Due to which equal priority threads behind current thread
in ready_q are not getting chance to run and leading to starvation.

This patch handles time slice in _ExcExit section context switch is
required.

Jira: ZEP-2444

Signed-off-by: Youvedeep Singh <youvedeep.singh@intel.com>
Signed-off-by: Ramesh Thomas <ramesh.thomas@intel.com>
Signed-off-by: Anas Nashif <anas.nashif@intel.com>