mesa: refuse to compile SPIR-V shaders or link mixed shaders

mesa: refuse to compile SPIR-V shaders or link mixed shaders

Note that gl_shader::CompileStatus will also indicate whether a shader
has been successfully specialized.

v2: Use the 'spirv_data' member of gl_shader to know if it is a SPIR-V
   shader, instead of a dedicated flag. (Timothy Arceri)

v3: Use bool instead of GLboolean. (Ian Romanick)

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>