audio/openal-soft: set LLD_UNSAFE

Linking with LLD currently fails due to preemption of symbols with
protected visibility in a shared object:

/usr/bin/ld: error: cannot preempt symbol: alIsExtensionPresent

defined in libopenal.so.1.18.0
referenced by examples/altonegen.c

CMakeFiles/altonegen.dir/examples/altonegen.c.o:(main)

/usr/bin/ld: error: cannot preempt symbol: alcGetCurrentContext

defined in libopenal.so.1.18.0
referenced by examples/altonegen.c

CMakeFiles/altonegen.dir/examples/altonegen.c.o:(main)

/usr/bin/ld: error: cannot preempt symbol: alcGetContextsDevice

defined in libopenal.so.1.18.0
referenced by examples/altonegen.c

CMakeFiles/altonegen.dir/examples/altonegen.c.o:(main)

...

(Ports that depend on audio/openal-soft may also need to be tagged with
LLD_UNSAFE.)

PR: 219089
Approved by: kwm
Sponsored by: The FreeBSD Foundation