games/openastromenace: fix build when lld is the system linker

Ports that use openal (such as this one) fail to link with lld as the
system linker due to disagreement in the handling of preemption of
protected visibility symbols in shared objects.

Set LLD_UNSAFE to fall back to ld.bfd for now.

PR: 226980
Approved by: portmgr (LLD_UNSAFE blanket)
Sponsored by: The FreeBSD Foundation