.clang-format: disable DerivePointerAlignment

everything outside of third_party should follow 'PointerAlignment:
right' i.e., associate the '*' with the variable

+ add a note about the clang-format that generated this file

Change-Id: I13e3f4f5fb6e22a8fa7fc3d06879c995b7c41a39
(cherry picked from libvpx e429080)