x86, NUMA: rename srat_64.c to srat.c

Rename srat_64.c to srat.c.  This is to prepare for unification of
NUMA init paths between 32 and 64bit.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Yinghai Lu <yinghai@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>