loader: Only create gfx 4th bindings when gfx is available

Only create the gfx bindings for 4th when it's compiled into the
loader. We do this with a linker set that only gets brought in to those
loaders that call gfx_framework_init. This calls gfx_interp_md() will
will drag in gfx_loader.c which will add to the linker set that
registers these bindings.

Sponsored by: Netflix
Reviewed by: kevans, jhb
Differential Revision: https://reviews.freebsd.org/D43904