mesa: big update, and related improvements (close #6367)

* Remove upstreamed patch
* EGL driver dirs are no longer a thing, see mesa commit
  407450eb84f5a1b466a2eff19b85cdee7ac15dfb
  Patching runtime search dir in sources instead (by vcunat).
* static-gallium patch dropped as it causes visibility issues with
  nouveau
* rtti patch dropped as it the relevant configuration has been removed
  upstream; properly fixed by an LLVM flag now
* Checks disabled. This is known and solved upstream. Checks will be
  re-enabled in a future commit when updating to a version containing
  the fixes
* Use llvm-3.5 now (by vcunat).