GetTargetTriple.cmake: detect MinGW 64 bits.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@116424 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/cmake/modules/GetTargetTriple.cmake b/cmake/modules/GetTargetTriple.cmake
index ac0c009..f4321c9 100644
--- a/cmake/modules/GetTargetTriple.cmake
+++ b/cmake/modules/GetTargetTriple.cmake
@@ -9,7 +9,11 @@
       set( value "i686-pc-win32" )
     endif()
   elseif( MINGW AND NOT MSYS )
-    set( value "i686-pc-mingw32" )
+    if( CMAKE_SIZEOF_VOID_P EQUAL 8 )
+      set( value "x86_64-w64-mingw32" )
+    else()
+      set( value "i686-pc-mingw32" )
+    endif()
   else( MSVC )
     set(config_guess ${LLVM_MAIN_SRC_DIR}/autoconf/config.guess)
     execute_process(COMMAND sh ${config_guess}