Makefile.defs: enabled fast locks for amd64

src/Makefile.defs

@@ -788,6 +788,10 @@ ifeq ($(ARCH), x86_64)
 	use_fast_lock=yes
 endif
 
+ifeq ($(ARCH), amd64)
+	use_fast_lock=yes
+endif
+
 ifeq ($(ARCH), sparc64)
 ifeq 	($(CC_NAME), gcc)
 	use_fast_lock=yes