Remove runtime assembly generation

Now that all of the dispatch stubs are generated at compile time, there's a bunch of code that we can remove.

The first commit in this set just removes the now-unused functions for allocating executable memory.

The second is to remove all of the runtime code generation for the various platforms. At this point, the only thing that's used for is to restore the GL dispatch stubs after patching them. To do that, we can just use memcpy to make a copy of the original stubs before patching, and then copy it back to restore.

That way, we can just use the same code for every architecture, instead of having to maintain an extra assembly template.