radeonsi: implement ARB_texture_cube_map_array

No LLVM changes needed.

Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>

v2: updated GL3.txt and relnotes

docs/relnotes/10.3.html

@@ -45,6 +45,7 @@ Note: some of the new features are only available with certain drivers.
 
 <ul>
 <li>GL_ARB_stencil_texturing on nv50, nvc0, r600, and radeonsi</li>
+<li>GL_ARB_texture_cube_map_array on radeonsi</li>
 </ul>