Also set LLD_UNSAFE when using BINARY_ALIAS to choose ld.bfd

arm64 provides no /usr/bin/ld.bfd by default. LLD_UNSAFE automatically
adds a dependency on binutils for this case, so is needed in addition
to BINARY_ALIAS=ld=ld.bfd.

Approved by: portmgr (LLD_UNSAFE blanket)
Sponsored by: The FreeBSD Foundation