Makefile.defs: override the ARCH for amd64 to x86_64

src/Makefile.defs

@@ -377,7 +377,7 @@ endif
 
 # retarget amd64 to x86_64
 ifeq ($(ARCH), amd64)
-	ARCH=x86_64
+	override ARCH=x86_64
 endif
 
 $(info target architecture <$(ARCH)>, host architecture <$(HOST_ARCH)>)