MFC: r273754 (by nwhitehorn)

Use pkg-1.4-style platform identifiers based on MACHINE_ARCH (e.g.
FreeBSD:11:amd64 instead of freebsd:11:x86:64) when bootstrapping pkg.
Thanks to portmgr for providing symlinks so both styles work.

Reviewed by: bapt
MFC after: 3 weeks