[S390] Cleanup xtime usage

This replaces direct xtime usage in the s390 arch with timekeeping accessors,
so we can further clean up the timekeeping core.

Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: John Stultz <johnstul@us.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>