The ACPI parts are identical between i386 and amd64

Apart from one MD file, ACPI is a x86 implementation, not specific to either
i386 or amd64, so put it into files.x86. Other architectures include fewer
files for the same options, so it can't move into the MI files file.

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