x86/Kconfig: Make SPARSEMEM default for 32-bit x86

Sparsemem has been a default memory model for x86-64 for over a decade,
since:

  b263295d

 ("x86: 64-bit, make sparsemem vmemmap the only memory model").

Make it the default for 32-bit NUMA systems (if there any left) as well.

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: Christoph Hellwig <hch@infradead.org>
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/1556112252-9339-2-git-send-email-rppt@linux.ibm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>