Currently there is a hard-coded conflation of "value classes" and "preview mode resources" in the Valhalla Make files.

It would be much clearer for future maintainers if this were split, so that the changes related only to "preview mode" in general (such as compiling preview classes) were more suitably named.

This should be a very safe change, since any mismatch in naming will cause obvious build issues.

This work should be done before JDK-8377101, since that will also affect Makefiles in the same area.