[IA64] ia64: simplify and fix udelay()

The original ia64 udelay() was simple, but flawed for platforms without
synchronized ITCs:  a preemption and migration to another CPU during the
while-loop likely resulted in too-early termination or very, very
lengthy looping.

The first fix (now in 2.6.15) broke the delay loop into smaller,
non-preemptible chunks, reenabling preemption between the chunks.  This
fix is flawed in that the total udelay is computed to be the sum of just
the non-premptible while-loop pieces, i.e., not counting the time spent
in the interim preemptible periods.  If an interrupt or a migration
occurs during one of these interim periods, then that time is invisible
and only serves to lengthen the effective udelay().

This new fix backs out the current flawed fix and returns to a simple
udelay(), fully preemptible and interruptible.  It implements two simple
alternative udelay() routines:  one a default generic version that uses
ia64_get_itc(), and the other an sn-specific version that uses that
platform's RTC.

Signed-off-by: John Hawkes <hawkes@sgi.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>