Move get_extension_function_address() method to platform

This moves the get_extension_function_address() method from
the system class to the platform class. Also changes the method
to use the clGetExtensionFunctionAddressForPlatform() function
instead of the deprecated clGetExtensionFunctionAddress() when
compiling with OpenCL 1.2.
Move unload_compiler() method to platform

This moves the unload_compiler() method from the system class
to the platform class. Also changes the method to use the
clUnloadPlatformCompiler() function instead of the deprecated
clUnloadCompiler() when compiling with OpenCL 1.2.