Respect predefined defaults for AR, AS, LD and RANLIB

Makefile.system

@@ -263,10 +263,10 @@ endif
 
 ARFLAGS	=
 CPP	= $(COMPILER) -E
-AR	= $(CROSS_SUFFIX)ar
-AS	= $(CROSS_SUFFIX)as
-LD	= $(CROSS_SUFFIX)ld
-RANLIB	= $(CROSS_SUFFIX)ranlib
+AR	?= $(CROSS_SUFFIX)ar
+AS	?= $(CROSS_SUFFIX)as
+LD	?= $(CROSS_SUFFIX)ld
+RANLIB	?= $(CROSS_SUFFIX)ranlib
 NM	= $(CROSS_SUFFIX)nm
 DLLWRAP = $(CROSS_SUFFIX)dllwrap
 OBJCOPY = $(CROSS_SUFFIX)objcopy