GLdispatch: Define dynamic dispatch stubs at compile time.

Instead of allocating pages of memory for the dynamic dispatch stubs, define
them at build time like the static stubs.

The dynamic stubs are identical to the static ones, so we can use the same code
to define both the static and dynamic stubs at compile time.

Removed entry_generate(). entry_generate_default_code is now only used to
restore entrypoints after patching.

Aside from simplifying things by not having a separate allocation for the
dynamic stubs, this should also allow the dispatch stubs to function properly
in both processes after a call to fork.