PPC: machdep, zero BSS always but BookE

Zero BSS always. The only case when this operation is
ommitted is when booting on BookE.

Created by: Wojciech Macek <wma@semihalf.com>
Obtained from: Semihalf
Reviewed by: imp, nwhitehorn
Differential revision: https://reviews.freebsd.org/D13948
Sponsored by: QCM Technologies