Add x86-64 as a Tier 3 platform: we're currently able to boot the i386
FreeBSD implementation on it at production quality, and x86-64-specific
code is in progress in a combination of the main FreeBSD CVS repository
and Perforce. Once x86-64 native support is appropriately improved, we
can migrate it to Tier 2.

Requested by: obrien
Approved by: core