hwcontext_opencl: DRM to OpenCL mapping for ARM

Using cl_arm_import_memory.  Unfortunately, despite this not being a
standard extension, the function clImportMemoryARM() is not accessible
via clGetExtensionFunctionAddressForPlatform().  This means that it has
to be linked directly to the ARM OpenCL binary, so making a portable
binary is not possible as it is with all other mapping extensions.