emit dynamically linked 6.out on darwin instead of static,

emit dynamically linked 6.out on darwin instead of static,
unless given -d flag.

the resulting binary doesn't *use* any dynamic libraries,
it just has a reference to the dynamic linker and an
empty list of needed symbols and libraries.

this is enough to make 6.out binaries that can be run
under dtrace / dtruss.

R=r
DELTA=39  (34 added, 0 deleted, 5 changed)
OCL=20476
CL=20482