support for more than 32 CPUs

[Manuel Bouyer <bouyer%antioche.eu.org@localhost>]

Hello,
the attached patch, based on a patch sent by Mindaugas Rasiukevicius
on tech-kern@ some time ago, bumps the max number of CPUs to 256 for
amd64, and should easily allow up to 64 for Xen/amd64. I tested it
on a x86 with 64 AMD cores (lighly as this box has now known drive yet - some
driver hacking is needed), 8 intel cores and with a Xen domU with 4 core.
I didn't notice regressions so far.

Comments before I commit ?

-- 
Manuel Bouyer <bouyer%antioche.eu.org@localhost>
     NetBSD: 26 ans d'experience feront toujours la difference
--