Switch AMD64 and i386 platforms to using ELF as their kernel crash

Switch AMD64 and i386 platforms to using ELF as their kernel crash
dump format.  The key reason to do this is so that we can dump sparse
address space.  For example, we need to be able to skip the PCI hole
just below the 4GB boundary.  Trying to destructively dump MMIO device
registers is Really Bad(TM).  The frequent result of trying to do a
crash dump on a machine with 4GB or more ram was ugly (lockup or reboot).

This code has been taken directly from the IA64 dump_machdep.c code,
with just a few (mostly minor) mods.

Introduce a dump_avail[] array in the machdep.c code so that we have a
source of truth for what memory is present in a machine that needs to be
dumped.  We can't use phys_avail[] because all sorts of things slice
memory out of it that we really need to dump.  eg: the vm page array
and the dmesg buffer.  dump_avail[] is pretty much an unmolested version
of phys_avail[].  It does have Maxmem correction.

Bump the i386 and amd64 dump format to version 2, but nothing actually
uses this.  amd64 was actually using the i386 dump version number.

libkvm support to follow.

Approved by:	re