um: Convert to use read_persistent_clock

This patch converts the um arch to use read_persistent_clock().
This allows it to avoid accessing xtime and wall_to_monotonic
directly.

Signed-off-by: John Stultz <johnstul@us.ibm.com>
Cc: Jeff Dike <jdike@addtoit.com>
LKML-Reference: <1279068988-21864-8-git-send-email-johnstul@us.ibm.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>