scons: Prefer x86_64-w64-mingw32- prefix.

This allows to use mingw-w64 binaries on debian systems which already
include a (typically incomplete) 64 cross compiler.
diff --git a/scons/crossmingw.py b/scons/crossmingw.py
index 4a695a4..949fc4f 100644
--- a/scons/crossmingw.py
+++ b/scons/crossmingw.py
@@ -57,10 +57,10 @@
     i686-w64-mingw32-
 """)
 prefixes64 = SCons.Util.Split("""
+    x86_64-w64-mingw32-
     amd64-mingw32-
     amd64-mingw32msvc-
     amd64-pc-mingw32-
-    x86_64-w64-mingw32-
 """)
 
 def find(env):