x86: add X86_RESERVE_LOW_64K

This bugzilla:

  http://bugzilla.kernel.org/show_bug.cgi?id=11237

Documents a wide range of systems where the BIOS utilizes the first
64K of physical memory during suspend/resume and other hardware events.

Currently we reserve this memory on all AMI and Phoenix BIOS systems.
Life is too short to hunt subtle memory corruption problems like this,
so we try to be robust by default.

Still, allow this to be overriden: allow users who want that first 64K
of memory to be available to the kernel disable the quirk, via
CONFIG_X86_RESERVE_LOW_64K=n.

Also, allow the early reservation to overlap with other
early reservations.

Signed-off-by: Ingo Molnar <mingo@elte.hu>