Implement ArGetStackPointer() routine

xtoskrnl/includes/amd64/ar.h

@@ -21,6 +21,11 @@ XTCDECL
 BOOLEAN
 ArCpuId(IN OUT PCPUID_REGISTERS Registers);
 
+XTASSEMBLY
+XTCDECL
+ULONG_PTR
+ArGetStackPointer();
+
 XTCDECL
 VOID
 ArHalt();