Revert r314669, r314670:
Bring back the i486 option in GENERIC by default.

The code related to i386 CPU variants configuration has received many
changes in the last years: most of the features are detected automatically,
so there are no performance penalties from keeping the 486 support enabled.

Re-instate the 486 support: while the general configuration could still be
cleaned a bit, there is no advantage in removing it.

Differential Revision: https://reviews.freebsd.org/D9879