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