GLX: Define the GLX dispatch stubs at compile time.

Rewrote the generated dispatch stubs for GLX so that they're all defined at
compile time, instead of generating them at runtime.

Insted of patching the address of the vendor-provided functions into the
assembly stubs, it will now keep a separate array of function pointers. The
assembly stubs will look up the approprite entry in that array and jump to it.

As a result, the assembly code is entirely static, so we don't need to deal
with generating or modifying executable code at runtime.