Align offline compilation with cl21_trunk (#400)

Add a special case for the Khronos compiler and always use it for
OpenCL C++, otherwise use the same logic as on the cl21_trunk
branch.

The CMake option to pass the path to the khronos compiler has
been renamed to KHRONOS_OFFLINE_COMPILER_OPTIONS and is no longer
mandatory.

Signed-off-by: Kevin Petit <kevin.petit@arm.com>