Update license notice.

src/etc/third-party/README.txt

@@ -1,8 +1,5 @@
-Certain files in this distribution are covered by a different license than the rest of the Rust Project.
-Specifically:
+Certain binaries in this distribution do not originate from the Rust project, but are distributed with it in its binary form. These binaries, including gcc and other parts of the GNU compiler toolchain, are licensed either under the terms of the GNU General Public License, or the GNU General Public License with the GCC Runtime Library Exception, as published by the Free Software Foundation, either version 3, or (at your option) any later version. See the files COPYING3 and COPYING.RUNTIME respectively.
 
-   - libgcc_s_dw2-1.dll and libstdc++6.dll are distributed under the terms of the GNU General Public License 
-     with the GCC Runtime Library Exception as published by the Free Software Foundation; either version 3, 
-     or (at your option) any later version.  See the files COPYING3 and COPYING.RUNTIME respectively.
-     You can obtain a copy of the source of these libraries either here: http://sourceforge.net/projects/mingw/files/MinGW/Base/gcc/Version4/gcc-4.5.2-1/,
-     or from the project website at http://gcc.gnu.org
+You can obtain a copy of the source of these libraries from the MinGW-w64 project[1].
+
+[1]: http://mingw-w64.sourceforge.net/