Makefile.defs: explicit retarget of amd64 arch to x86_64

- it is supposed to be done by the arch detection scripts, but it may
  fail on various systems having a different output than expected

src/Makefile.defs

@@ -374,6 +374,12 @@ ARCH:=$(CC_ARCH)
 else
 ARCH:=$(HOST_ARCH)
 endif
+
+# retarget amd64 to x86_64
+ifeq ($(ARCH), amd64)
+	ARCH=x86_64
+endif
+
 $(info target architecture <$(ARCH)>, host architecture <$(HOST_ARCH)>)
 
 LIBDIR ?=
@@ -788,10 +794,6 @@ 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