loader: rename gfx_interp_md to gfx_interp_ref

We have the call to gfx_interp_ref to bring in the .o so that we get the
linker set item to add the language bindings at the right time. Where we
call it is not the right time... So the _ref name is better. Change it
before we have too many others like it.

Sponsored by: Netflix

(cherry picked from commit 6faf55c86d9b86f40de8d19970e42cb42a892c47)