x86: allow NODES_SHIFT to be a config option on x86_64

Allow the maximum number of nodes in an x86_64 system to
be configurable. This patch does NOT change the default value
but allows the value to be a config option.

Signed-off-by: Jack Steiner <steiner@sgi.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>