Linux calls it a sparc64, not a sun4u.

src/build/Common.gmake

@@ -168,6 +168,9 @@ endif
 os      :=	$(shell echo $(os) | tr '[A-Z]' '[a-z]' | tr ' ' '_')
 
 machine		:=	$(shell if uname -m | egrep -v "(i[3456789]|x)86" > /dev/null; then uname -m | tr "[A-Z]" "[a-z]" | tr " " "_" ; fi)
+ifeq ($(machine),sparc64)
+ CFLAGS +=      -D__BIG_ENDIAN__
+endif
 ifeq ($(machine),sun4u)
  CFLAGS +=      -D__BIG_ENDIAN__
 endif