x86: make sparsemem more available

With CONFIG_X86_PC, I can set CONFIG_SPARSEMEM=y.

With CONFIG_X86_GENERICARCH, CONFIG_SPARSEMEM depends on CONFIG_NUMA.

I'm using the patch below to enable sparsemem instead of flatmem.
System booted and is running.

Signed-off-by: Ingo Molnar <mingo@elte.hu>