support x86_64-windows;

--- a/NEWS	Mon Aug 31 22:44:07 2015 +0200
+++ b/NEWS	Mon Aug 31 22:45:40 2015 +0200
@@ -307,8 +307,8 @@
 
 *** System ***
 
-* Poly/ML 5.5.3 runs natively on x86-windows, with somewhat larger heap
-space than former x86-cygwin.
+* Poly/ML 5.5.3 runs natively on x86-windows and x86_64-windows,
+which both allow larger heap space than former x86-cygwin.
 
 * Java runtime environment for x86_64-windows allows to use larger heap
 space.