Use BINARY_ALIAS to make LLD_UNSAFE more widely applicable

Some ports do not honour $LD or -fuse-ld=bfd in $CFLAGS, but do invoke
ld via $PATH. Automatically set BINARY_ALIAS+=ld=${LD} when LLD_UNSAFE
is active to use ld.bfd for these ports.

Approved by: portmgr (antoine)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D14876