lang/mono: set LLD_UNSAFE to avoid linking with lld

mono fails to link with lld because lld defaults to -ztext and does not
allow relocations in readonly segments (such as the one containing
.text). For now just fall back to linking with ld.bfd if the system ld
is lld.

PR: 214864
Approved by: portmgr (LLD_UNSAFE blanket)
Sponsored by: The FreeBSD Foundation