xgl-generate.py: add LayerGetProcAddrHelperSubcommand()
xgl-generate.py: add LayerGetProcAddrHelperSubcommand()

It generates layer_intercept_proc() to intercept all but
xglInitAndEnumerateGpus().  It is possible to extend the subcommand to get the
list of functions to intercept from command arguments or layer spec files.
But we do not have the needs yet.

KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools