Only change the fadt revision in acpidump on i386

There is a workaround in acpidump for an old IBM laptop. This is being
hit on a modern arm64 system causing acpidump to use the wrong field
when reading a later address.

As the laptop has a 32-bit CPU the only FreeBSD support for it is i386.
As such limit the workaround to that architecture.

Reviewed by: emaste, jkim, jhb (all earlier version)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D32573