The source engine, the engine which Valve have used for most of their games (Some older games have been imported into source), supports android. Particularly, Portal and Half Life 2 have been ported over to run on the Nvidia android device line up. The CPU that Nvidia use is based upon the ARMv7 Architecture, the same as that of the Pi.
Now, here's the catch, Valve have to compile a version of <Game> in order for us to play it. As we don't have the source code we can't do that ourself. Sure, you could install android and all hat, but even then, Portal and Half Life 2 are built to run with Nvidia's GPU.