Cleanup clock_*time includes

Clock_gettime, settime and getres implementations are unncessarily
complex due to using defines and C file inclusion.  Simplify the
code by replacing the redundant defines and removing the inclusion,
making it much easier to understand.  No functional changes.

	* sysdeps/posix/clock_getres.c (__clock_getres): Cleanup.
	* sysdeps/unix/clock_gettime.c (__clock_gettime): Cleanup.
	* sysdeps/unix/clock_settime.c (__clock_settime): Cleanup.
	* sysdeps/unix/sysv/linux/clock_getres.c (__clock_getres): Cleanup.
	* sysdeps/unix/sysv/linux/clock_gettime.c (__clock_gettime): Cleanup.
	* sysdeps/unix/sysv/linux/clock_settime.c (__clock_settime): Cleanup.