kboot: Split out powerpc build

Split the powerpc specific parts into kboot/arch/powerpc64. More may be
needed here.

Sponsored by: Netflix
Reviewed by: jhibbits
Differential Revision: https://reviews.freebsd.org/D33514

(cherry picked from commit 6497250f6f7f93e36795f5d0ec8ac658f6e781d8)