ARB_gpu_shader_int64 is about supporting 64-bit scalar and vector integer data types for shaders and related 64-bit integer support for OpenGL. The Intel i965 Mesa driver has supported INT64 for Broadwell and newer, RadeonSI Gallium3D also has supported INT64, and there's been support in LLVMpipe/Softpipe too, while now it's been implemented for Nouveau.
However, as Mirkin explained, for now it's only being flipped on for pre-Maxwell hardware. "Everything should be ready to go on maxwell as well, but the sched code calculator ends up causing a lot of failures with the generated code. Since this is a fringe feature, leave it out until the sched code logic is improved."
The code for now can be found via Mesa-dev but should end up hitting Mesa Git soon enough, marking another feature for Mesa 17.1.