GLdispatch: Remove runtime assembly code generation.

Removed the rest of the assembly code generation from libGLdispatch. Since we
don't need to generate dispatch stubs at runtime for extension functions
anymore, the code generation is now only used to restore the dispatch stubs
after a vendor library patches them.

Now, instead of reconstructing each dispatch stub, libGLdispatch will just
allocate a copy of all of the entrypoints before patching them, and then it
uses memcpy to restore them.