x86/Kconfig: Deprecate DISCONTIGMEM support for 32-bit x86

Mel Gorman says:

  "32-bit NUMA systems should be non-existent in practice.  The last NUMA
   system I'm aware of that was both NUMA and 32-bit only died somewhere
   between 2004 and 2007. If someone is running a 64-bit capable system in
   32-bit mode with NUMA, they really are just punishing themselves for fun."

Mark DISCONTIGMEM broken for now as suggested by Christoph Hellwig,
and (hopefully) remove it in a couple of releases.

Suggested-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
Acked-by: Mel Gorman <mgorman@techsingularity.net>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rik van Riel <riel@surriel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-mm@kvack.org
Link: http://lkml.kernel.org/r/1556112252-9339-3-git-send-email-rppt@linux.ibm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>