Mk/bsd.port.mk: detect powerpc64 abi

We'll have a new abi on ppc64 soon (ELFv2) which is incompatible with the ELFv1 abi. We need to detect the abi on which we build stuff.

Submitted by: mikael_urankar@gmail.com
Approved by: portmgr (earlier version)
Differential Revision: https://reviews.freebsd.org/D22039