x86: relax restrictions on setting CONFIG_NUMA on x86, #2

The FLATMEM memory model references a global mem_map and max_mapnr. This
is incompatible with how memory models used for NUMA view the world.
Builds fail if FLATMEM && NUMA are set on x86. This patch forbids that
combination of config items. This is consistent with x86_64
enforcements.

Signed-off-by: Mel Gorman <mel@csn.ul.ie>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>