scons: Add support for Mingw64.

scons/crossmingw.py

 import SCons.Util
 
 # This is what we search for to find mingw:
-prefixes = SCons.Util.Split("""
+prefixes32 = SCons.Util.Split("""
     mingw32-
     mingw32msvc-
     i386-mingw32-
     i686-mingw32msvc-
     i686-pc-mingw32-
 """)
+prefixes64 = SCons.Util.Split("""
+    amd64-mingw32-
+    amd64-mingw32msvc-
+    amd64-pc-mingw32-
+""")
 
 def find(env):
+    if env['machine'] == 'x86_64':
+        prefixes = prefixes64
+    else:
+        prefixes = prefixes32
     for prefix in prefixes:
         # First search in the SCons path and then the OS path:
         if env.WhereIs(prefix + 'gcc') or SCons.Util.WhereIs(prefix + 'gcc'):