uml: kill x86_64 STACK_TOP_MAX

The x86_64 a.out.h got a definition of STACK_TOP_MAX, which interferes with
the UML version.  So, just undef it like STACK_TOP.
Signed-off-by: NJeff Dike <jdike@linux.intel.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

include/asm-um/a.out.h

@@ -5,6 +5,7 @@
 #include "choose-mode.h"
 
 #undef STACK_TOP
+#undef STACK_TOP_MAX
 
 extern unsigned long stacksizelim;