Create files.x86

files.x86 is for the parts of the system that are common to both i386 and amd64
due too their nature. First up, to get the ball rolling, is fdc, the floppy disk
support. It works only on amd64 and i386 these days, and that's unlikely to
change.

Reviewed by: jhb, cem (earlier versrions)
Differential Revision: https://reviews.freebsd.org/D21210