linker: Link built-in functions instead of including them in every shader

linker: Link built-in functions instead of including them in every shader

This is an invasive set of changes.  Each user shader tracks a set of other
shaders that contain built-in functions.  During compilation, function
prototypes are imported from these shaders.  During linking, the
shaders are linked with these built-in-function shaders just like with
any other shader.