Add a USE_DISPATCH_ASM macro to config.h.

Define a new USE_DISPATCH_ASM macro in config.h if assembly dispatch stubs are
enabled. Update the GLX entrypoint generation to use that macro instead of
checking all of the USE_*_ASM macros.

This is less likely to break if we add assembly support for another
architecture. In addition, it'll make it easier to skip any unit tests
that depend on being able to generate new dispatch stubs.