Fix handling of .o files in LOCAL_GENERATED_SOURCES.

Rule-generated .o files (in gen_o_objects) were being given a dependency
on everything in LOCAL_GENERATED_SOURCES (except for other .o files);
unfortunately this can still create cycles in cases where there are
explicit dependencies between entries in LOCAL_GENERATED_SOURCES.

Instead, make handling of generated .o files consistent with other
generated files (which don't automatically get any dependencies on other
generated files) by excluding them from the target side of the rule.

Change-Id: I3fb5652dc3d85012c179a03b81887d16a85ab3bf