tu: Create singleton "empty" shaders

tu: Create singleton "empty" shaders

The most important of these is the empty FS, which should cut down on
time spent in the compiler when we fast-link a pipeline where there is
only a library with the VS and no library with the empty FS. Source
engine is known to do this.

This is also necessary for shader objects where the "empty" shaders are
never created up-front. We will use these when a NULL shader is bound.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25276>