x86: duplicate acpi_wakeup.c per i386 and amd64

The file as is is the maze of #ifdef passages, all slightly different.
Divorcing i386 and amd64 version actually makes changing the code
easier, also no changes for i386 are planned.

Reviewed by: markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D31931