mesa: Implement glSpecializeShaderARB

mesa: Implement glSpecializeShaderARB

v2:
  * Use gl_spirv_validation instead of spirv_to_nir.  This method just
    validates the shader. The conversion to NIR will happen later,
    during linking. (Alejandro Piñeiro)
  * Use gl_shader_spirv_data struct to store the SPIR-V data.
    (Eduardo Lima)
  * Use the 'spirv_data' member to tell if the gl_shader is a SPIR-V
    shader, instead of a dedicated flag. (Timothy Arceri)

Signed-off-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Signed-off-by: Alejandro Piñeiro <apinheiro@igalia.com>
Signed-off-by: Eduardo Lima Mitev <elima@igalia.com>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>