kernel: priority queues: declare as static inlines

After the move to C files we got some drop in the performance when
running latency_measure. This patch declares the priority queue
functions as static inlines with minor optimizations.

The result for one metric (on qemu):

3.6 and before the anything was changed:

  Get data from LIFO (w/ ctx switch): 13087 ns

after original change (46484da5):

  Get data from LIFO (w/ ctx switch): 13663 ns

with this change:

  Get data from LIFO (w/ ctx switch): 12543 ns

So overall, a net gain of ~ 500ns that can be seen across the board on many
of the metrics.

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