mesa: implement SPIR-V loading in glShaderBinary

mesa: implement SPIR-V loading in glShaderBinary

v2: * Add a gl_shader_spirv_data member to gl_shader, which already
   encapsulates a gl_spirv_module where the binary will be saved.
   (Eduardo Lima)

    * Just use the 'spirv_data' member to know whether a gl_shader has
   the SPIR_V_BINARY_ARB state. (Timothy Arceri)

    * Remove redundant argument checks. Move extension presence check
   to API entry point where the rest of checks are. Retype 'n' and
   'length'arguments to use the correct and more standard types.
   (Ian Romanick)

    * Fix some nitpicks. (Ian Romanick)

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