Add support for multiple OpenCL platforms (#1345)

cmake/config.cmake

cmake/modules/OpenCL.cmake

include/tvm/runtime/c_runtime_api.h

python/tvm/_ffi/runtime_ctypes.py

python/tvm/contrib/sdaccel.py

src/codegen/codegen_vhls.cc

src/codegen/opt/build_sdaccel_off.cc

src/pass/verify_memory.cc

src/runtime/c_runtime_api.cc

src/runtime/opencl/opencl_common.h

src/runtime/opencl/opencl_device_api.cc

src/runtime/opencl/opencl_module.cc

src/runtime/opencl/opencl_module.h

src/runtime/opencl/sdaccel/sdaccel_common.h

src/runtime/opencl/sdaccel/sdaccel_device_api.cc

src/runtime/opencl/sdaccel/sdaccel_module.cc

src/runtime/opencl/sdaccel/sdaccel_module.h