Add LLD_UNSAFE to force use of ld.bfd when ld is lld

By default the compiler will find /usr/bin/ld for the linker. If a port
sets LLD_UNSAFE and /usr/bin/ld is ld.lld, then add -fuse-ld=bfd to
force the use of the binutils BFD linker (from the base system, if
available, or by setting USE_BINUTILS).

This change is based on an earlier, similar patch from bapt, with the
addition of the -fuse-ld flag.

PR: 214864
Reviewed by: bapt
Approved by: portmgr (bapt)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D12064