x86, 64-bit: split x86_64_start_kernel

Split x86_64_start_kernel() into two pieces:

   The first essentially cleans up after head_64.S.  It clears the
   bss, zaps low identity mappings, sets up some early exception
   handlers.

   The second part preserves the boot data, reserves the kernel's
   text/data/bss, pagetables and ramdisk, and then starts the kernel
   proper.

This split is so that Xen can call the second part to do the set up it
needs done.  It doesn't need any of the first part setups, because it
doesn't boot via head_64.S, and its redundant or actively damaging.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Cc: xen-devel <xen-devel@lists.xensource.com>
Cc: Stephen Tweedie <sct@redhat.com>
Cc: Eduardo Habkost <ehabkost@redhat.com>
Cc: Mark McLoughlin <markmc@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>