lang/rexx-regutil: set LLD_UNSAFE to avoid linking with lld

lld does not have built-in search paths, so direct link invocations that
specify a library (e.g. -lncurses) but do not specify a search path
(e.g. -L/usr/lib) will fail.

PR: 214864
Submitted by: krion
Approved by: portmgr (LLD_UNSAFE blanket)