glsl/shader_cache: handle SPIR-V shaders

glsl/shader_cache: handle SPIR-V shaders

Right now we don't have cache support for SPIR-V shaders (from
ARB_gl_spirv). Right now they are properly skipped because they fall
on the ff shader code path (no key, no name), but it would be better
to update current comments, and add some guards.

Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>