[PowerPC] Fix powerpc 32 bit build in mmu_oea64.c

Due to ppc32 building mmu_oea64.c (for use when in bridge mode on a G5), we
need to guard the new moea64_page_array_startup code behind powerpc64
to avoid a compile error, since vm_offset_t is not 64-bit on ppc32.

Reviewed by: jhibbits
Differential Revision: https://reviews.freebsd.org/D22782