Mark i386 as Tier 2 starting in FreeBSD 13.0.

i386 was the first platform supported by FreeBSD and for a long
time was the dominant platform. However, it has been supplanted
for many years now by amd64 (64-bit x86). i386 now more closely
resembles other Tier 2 platforms in terms of userbase and active
development.

Reviewed by: emaste, imp, ygy
Differential Revision: https://reviews.freebsd.org/D26381