Added arch_cpu.cpp to the x86_64 build.

* Some things are currently ifndef'd out completely for x86_64 because
  they aren't implemented, there's a few other ifdef's to handle x86_64
  differences but most of the code works unchanged.
* Renamed some i386_* functions to x86_*.
* Added a temporary method for setting the current thread on x86_64
  (a global variable, not SMP safe). This will be changed to be done
  via the GS segment but I've not implemented that yet.

headers/os/kernel/OS.h

@@ -645,7 +645,7 @@ typedef enum cpu_types {
 
 #define B_CPU_x86_VENDOR_MASK	0xff00
 
-#ifdef __INTEL__
+#if defined(__INTEL__) || defined(__x86_64)
 typedef union {
 	struct {
 		uint32	max_eax;