Prevent BSS from being cleared twice on BookE

Summary:
First time BSS is cleared in booke_init(), Second time it's cleared in
powerpc_init(). Any variable initialized between two those guys gets wiped out
what is wrong. In particular it wipes tlb1_entries initialized by tlb1_init(),
which was fine when tlb1_init() was called a second time, but this was removed
in r304656.

Submitted by: Ivan Krivonos <int0dster_gmail.com>
Differential Revision: https://reviews.freebsd.org/D7638