Update some nbnxm kernel constants to constexpr

Update some nbnxm kernel constants to constexpr

Then OpenCL kernels now use preprocessor constants of the same names
and values, which are set up on the JIT compilation line.  This
eliminates a header file used for compatibility.

Added support also for the ocl_nbnxm_kernels testing target.

Change-Id: Ibf8f7864dd08b14aeecad095cdbe25b1fbacc765