loader: narrow the scope of gfx frame buffer wrt tg supported kernels

Store whether or not we found a vbefb module (eg, a tg supported kernel)
in the preloaded_file structure. This automatically resets on reload and
eliminates load_elf knowing about any gfx_* interface. Restrict this to
i386, which is the only place it's used. Update libi386 to check in the
preloaded_file struct. Eliminate this from the teken_gfx
structure. Rewrite the parsing code to be more inline. Check this from
the same place we check for a relocatable amd64 kernel.

Sponsored by: Netflix
Reviewed by: manu, tsoome
Differential Revision: https://reviews.freebsd.org/D33427