LJ combination rule kernels for OpenCL

LJ combination rule kernels for OpenCL

The current implementation enables combination rules for both AMD and
NVIDIA OpenCL (also ports the changes to the "nowarp" test/CPU kernel).

Like in the CUDA implementation, all kernels support it, but only for
plain cut-off are combination rules used.

Notes:
- On AMD tested on Hawaii, Fiji, Spectre and Oland devices;
  combination rules in all cases improve performance, although combined
  with the i-prefetching, the improvement is typically only ~10%.
- On NVIDIA tested on Kepler and Maxwell; in most cases the combination
  rule kernels are fastest.
  However, with certain inputs these kernels are 25% slower on Maxwell
  (e.g. pure water box, cut-off LJ, pot shift), but not on Kepler.
  This is likely a compiler mis-optimization, so we'll just leave the
  defaults the same as AMD.

Change-Id: I05396e000cdf93c1d872729e6b477192af152495