linker: Reject shaders that use too many varyings

linker: Reject shaders that use too many varyings

Previously it was up to the driver or later code generator to reject
these shaders.  It turns out that nobody did this.

This will need changes to support geometry shaders.

NOTE: This is a candidate for the stable branches.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=37743
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>