Even with a 16kB stack, we have been seeing stack overflows on PPC64
under stress.  This patch implements separate per-cpu stacks for
processing interrupts and softirqs, along the lines of the
CONFIG_4KSTACKS stuff on x86.  At the moment the stacks are still 16kB
but I hope we can reduce that to 8kB in future.  (Gcc is capable of
adding instructions to the function prolog to check the stack pointer
whenever it moves it downwards, and I want to use that when I try
using 8kB stacks so I can be confident that we aren't overflowing the
stack.)
Signed-off-by: Paul Mackerras <paulus@samba.org>