virtual/opencl: add dev-libs/ocl-icd as the fallback provider

virtual/opencl: add dev-libs/ocl-icd as the fallback provider

Advantages:
 * keyworded x86
 * supports multilib on amd64
 * all OpenCL-aware software should compile and link fine against it
 * should make it easier for user to deploy out-of-tree OpenCL
   providers (side note: all actual OpenCL providers currently
   in the tree except x11-drivers/nvidia-drivers actually *require*
   this to work), should they choose to do so

Disadvantages:
 * essentially a hack to ensure the integrity of the dependency tree
   on x86 and amd64/abi_x86_32 - most of the actual OpenCL providers
   currently in the tree are 64-bit only anyway

Signed-off-by: Marek Szuba <marecki@gentoo.org>