Allow truss to operate on ELFv2 processes.

Summary:
Currently, truss doesn't work on ELFv2 processes due to not recognizing the ABI.

Since there's no special handling needed here, just adding a PROCABI struct for
it is sufficient to get it working.

Submitted by: git_bdragon.rtk0.net
Differential Revision: https://reviews.freebsd.org/D18352