Modify ParserGen to produce bindings for x86_64-linux-gnu target.

Reference includes taken from Ubuntu 13.04 64-bit. For convenience, here's a zip that has the Linux headers bundled:
https://docs.google.com/file/d/0B7BkiMmXtP1lS0NpTEprU0UwLTA/edit
Just unpack it in some directory and change LINUX_INCLUDE_BASE_DIR on line 92 to point to it. For information, I pulled these headers from a few separate packages:
- libc6-dev
- linux-libc-dev
- libstdc++6-4.7-dev