Broken on master by f0f4e72c8ec5c08993ff.

Add the BIF type "gcbif" in bif.tab for defining GC BIFs. That will
eliminate some of the hand-written administrative code for handling
GC BIFs, saving the developer's time.

The make_tables script still contains a broken implementation
of a mechanism to automatically give a BIF an additional
(for example, was used for erlang:'++'/2 and erlang:append/2).
When that featured broke, it was worked around by adding
additional entries to bif.tab. There is therefore no reason
to mend the feature.