On certain newer Intel Atom based motherboards, for instance the
D2500CC which I have, syscons in text-mode fails to show the expected
contents due to write errors into video-memory.

At least one of the causes is that we copy from syscons internal buffer
to the video memory with optimized bcopy(9) which uses >16bit operations.

Until now, 32bit and wider operations have always worked on the video
memory, but since I cannot find a single source which says that this
SHALL work, and since these chipsets/bugs are now out there, this
commit changes syscons to always use 16bit copies on i386 & amd64.

This may be relevevant for PR's:
166262
166639
and various other bug reports floating elsewhere on the net, but
I lack hardware to test those.