Switch GNU ld to be installed as ld.bfd and linked as ld

We intend to replace GNU ld with LLVM's lld, and on the path to there
we'll experiment with having lld installed or linked as /usr/bin/ld.
Thus, make ld.bfd the primary install target for GNU ld, to later
facilitate making the ld link optional.

Reviewed by: davide, dim
Differential Revision: https://reviews.freebsd.org/D4790