kernel: sched: inline update_cache

This improves context switching by 7% when measured using the
thread_metric benchmark.

Before:
**** Thread-Metric Preemptive Scheduling Test **** Relative Time: 120
Time Period Total:  5451879

After:
**** Thread-Metric Preemptive Scheduling Test **** Relative Time: 30
Time Period Total:  5853535

Signed-off-by: Anas Nashif <anas.nashif@intel.com>