Move the identical x86 lines to files.x86

Move all the identical x86 lines to files.x86. The non-identical ones should be
unified and moved as well, but that would require additional changes that would
need a more careful review and may not be MFCable, so I'll do them
separately. I'll delete the mildly snarky comment when things are unified.

Reviewed by: jhb (verbal OK on irc)
Differential Revision: https://reviews.freebsd.org/D21248