Crash in firefox3 in thr_kill

[Justin Hibbits]

It seems that thr_kill doesn't behave properly when the VM is under load.  When
doing anything that stresses the hard disk, firefox dies in thr_kill, called
from pthread_mutex_lock.  The crash address is the bnslr immediately following
the sc, so I believe it's encountering some kind of page fault inside the
kernel.

Since I've only seen this on powerpc, I can only assume that it's specific to
this arch.