clang: enable GPU backends for the opencl DISTRO_FEATURE

clang: enable GPU backends for the opencl DISTRO_FEATURE

Clang gates enabling of several backends (AMDGPU, NVPTX and SPIR-V) by
the GPU-related distro features (opengl, vulkan). Now as we got a
separate feature for OpenCL, enable those backends for the OpenCL-only
configurations.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>