x86/Kconfig: Select ARCH_SELECT_MEMORY_MODEL only if FLATMEM and SPARSEMEM are possible

x86-64 supports only CONFIG_SPARSEMEM; there is nothing users can select.
So enable the memory model selection (via CONFIG_ARCH_SELECT_MEMORY_MODEL)
only if both, SPARSEMEM and FLATMEM are possible, which isn't the case
on x86-64.

Signed-off-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/20210929144321.50411-1-david@redhat.com