Update comments and simplify conditionals for compat32

Only amd64 (because of i386) needs 32-bit time_t compat now, everything else is
64-bit time_t. Rather than checking on all 64-bit time_t archs, only check the
oddball amd64/i386.

Reviewed By: emaste, kib, andrew
Differential Revision: https://reviews.freebsd.org/D11364