apm: Only install rc script on i386

The apm(8) rc script only works on i386, but it's installed on all
platforms. Only install it on i386, which avoids creating a useless
FreeBSD-apm package on other platforms.

While here, build the acpi package on i386.

MFC after: 2 weeks
Reviewed by: imp
Sponsored by: https://www.patreon.com/bsdivy
Differential Revision: https://reviews.freebsd.org/D56629