Rename kernel's clock_vars.h to sys_clock.h

The revised name better reflects the fact that the file defines
macros in addition to declaring variables. In addition, most of
these APIs use the "sys_clock_" prefix, which aligns well with
the new file name.

Change-Id: Ib33517d4b19ec2455303b87200c677e87640fcbc
Signed-off-by: Allan Stephens <allan.stephens@windriver.com>