Port for x64 GMod (x86-64 branch)

[Grocel]

When the GMod 64-bit build is released gm_bass3 will break, because many internal stuff (headers, etc.) is changed. It can't no longer use Source SDK 2013 to build. It will also break for the 32-bit version of the game.

https://forum.facepunch.com/f/gmoddev/btgwr/64-bit-Garry-s-Mod/1/

Perhaps I could make it work again after that by sacrificing the mounted valve file system support. It would no longer be able to load and playback content from gcfs, vpks, addons etc.