Copy src/third_party/mingw-w64 to deps/third_party/, update LICENSE and README

deps/third_party/mingw-w64/README.chromium

Index: deps/third_party/mingw-w64/README.chromium
===================================================================
--- deps/third_party/mingw-w64/README.chromium	(revision 7063)
+++ deps/third_party/mingw-w64/README.chromium	(working copy)
@@ -1,17 +1,19 @@
 URL: http://sourceforge.net/projects/mingw-w64/
 Version: 1.0
-License: LGPL
+License: GPL3
 License File: LICENSE
 
 Description:
 Provides toolchain binaries for building 64-bit Windows object files.
 
+Binaries found at http://sourceforge.net/projects/mingw-w64/files/
+
 Currently NaCl uses mingw-64's version of GNU "as" assembler because
 it seems to be the only gnu-compatible assembler capable of emitting
 64-bit COFF/PE+ object files.
 
-Since we are using only the compiled executable and no static libraries,
-the LGPL's linking requirements would appear to be met.
+Note we are just using this tool to do the build.
+We are not redistributing it.
 
 Local Modifications:
-No Modifications
+No Modifications