x86: fix freeze in x86_64 RTC update code in time_64.c

Fix hard freeze on x86_64 when the ntpd service calls 
update_persistent_clock()

A repeatable but randomly timed freeze has been happening in Fedora 6
and 7 for the last year, whenever I run the ntpd service on my AMD64x2
HP Pavilion dv9000z laptop.  This freeze is due to the use of
spin_lock(&rtc_lock) under the assumption (per a bad comment) that
set_rtc_mmss is called only with interrupts disabled.  The call from
ntp.c to update_persistent_clock is made with interrupts enabled.

Signed-off-by: David P. Reed <dpreed@reed.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>