From: "H. Peter Anvin" <hpa@zytor.com>

This patch cleans up the very early memory setup on the i386 platform.  In
particular, it removes the hard-coded 8 MB limit completely by dynamically
creating the early-boot pagetables rather than having them hard coded.

While I was at it, I changed head.S so that it always sets up a local GDT;
this means among other things that SMP and VISWS are no longer special
cases, and is conceptually cleaner to boot.  The VISWS people have
confirmed it works on VISWS.

It also uses a separate entrypoint for non-boot processors since this is
completely kernel-internal anyway.  This eliminates the need to set %bx on
boot.  (If you think this is a bad idea I can eliminate this change; it
just seemed cleaner to me to do it this way.)

Additionally, zero bss with rep;stosl rather that rep;stosb.