drop detection code for 386sx/386dx CPUs

drop detection code for 386sx/386dx CPUs

OpenBSD/i386 doesn't actually support running on 386sx or 386dx CPUs
so we don't need to test whether we're running on one of these CPUs
anymore.

The 486 (which was launched in 1989) added a few new features over
the 386:
- an alignment check flag in EFLAGS
- 3 new userland instructions: bswap / cmpxchg / xadd
- 3 new kernel mode instructions: invd / wbinvd / invlpg
- new bits in CR0 (386 CPUs did not support ring0 write protection)
- new bits in CR3

In this diff, we remove the code that checks for the alignment check
flag as we've only supported Pentium (or newer) CPUs for some time.

The rest of the diff is about updating comments related to pre-486
CPUs. 2 files under arch/amd64 are updated to keep them in sync
with the arch/i386 updates.

ok mlarkin@, jsg@; tweak from miod@