x86, kexec: Remove 1024G limitation for kexec buffer on 64bit

Now 64bit kernel supports more than 1T ram and kexec tools
could find buffer above 1T, remove that obsolete limitation.
and use MAXMEM instead.

Tested on system with more than 1024G ram.

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Link: http://lkml.kernel.org/r/1359058816-7615-22-git-send-email-yinghai@kernel.org
Cc: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>