Poudriere on powerpc64

[Justin Hibbits]

I just tried filing a ticket in the poudriere fossil, but hit a 'read-only
database' error, so next best would be this list.

The text of the ticket is (copy and paste):

When configuring a 'native' poudriere jail on powerpc64, it gets configured
for powerpc.powerpc.  The PowerPC FreeBSD port is different from most
others, because it has a target.target_arch, so it's powerpc.powerpc64
(target.target_arch), but poudriere misses the target_arch, and treats it
as powerpc.powerpc.

Tied with this, the '-a' argument, to specify a target architecture, errors
with 'only supported on amd64', and with a change for this ticket, it
should be expanded to support powerpc64 as well.

- Justin