ati-driver: Update to 4.12

Tested with a HD 7850 on a x86_64 system with the following kernels:
- 3.12
- 3.14
- 3.16
- 3.17
- 3.18

glxgears and minecraft works. The gentoo patch is no longer required. However, to support 3.17 (and 3.18) I added a small patch from arch linux.

see also #5266

@iElectric I think this should also be merged into 14.12 release branch, right?
cc @MarcWeber @offlinehacker