K8 has an erratum (#100) that essentially causes some compat mode processes
to fault occassionally. The issue can be worked around in the OS. It only
applies to x86-64, in 32bit it is fine.

This adds a check to the page fault handler that checks for addresses >4GB
from compat mode. If they happen just return; the CPU will reexecute
the instruction and the condition that caused the problem is gone.

More details in Opteron/Athlon64 specification update on the AMD website.

Also I removed a left over debugging printk in the prefetch handling code.