x86, kexec: Limit the crashkernel address appropriately

Keep the crash kernel address below 512 MiB for 32 bits and 896 MiB
for 64 bits.  For 32 bits, this retains compatibility with earlier
kernel releases, and makes it work even if the vmalloc= setting is
adjusted.

For 64 bits, we should be able to increase this substantially once a
hard-coded limit in kexec-tools is fixed.

Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
Cc: Vivek Goyal <vgoyal@redhat.com>
Cc: Stanislaw Gruszka <sgruszka@redhat.com>
Cc: Yinghai Lu <yinghai@kernel.org>
LKML-Reference: <20101217195035.GE14502@redhat.com>