Webkit2 doesnt build on powerpc 32 bits
https://bugs.webkit.org/show_bug.cgi?id=130837

Reviewed by Carlos Garcia Campos.

Check if libatomic is needed in order to use std::atomic, and add
it to the list of WebKit2 libraries.

* PlatformGTK.cmake:


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@176519 268f45cc-cd09-0410-ab3c-d52691b4dbfc