radeonsi: enable EXT_shader_image_load_store

This depends on LLVM 10 because this needs https://reviews.llvm.org/D65283

Reviewed-by: Marek Olšák <marek.olsak@amd.com>

docs/relnotes/19.2.0.html

@@ -40,6 +40,7 @@ TBD.
 
 <ul>
 <li>GL_ARB_post_depth_coverage on radeonsi (Navi)</li>
+<li>GL_EXT_shader_image_load_store on radeonsi (with LLVM >= 10)</li>
 <li>GL_EXT_shader_samples_identical on iris and radeonsi (if using NIR)</li>
 <li>EGL_EXT_platform_device</li>
 <li>VK_EXT_queue_family_foreign for radv</li>