API CHANGE: set_gcc_path removed from libLTO C binding

If you haven't written code to the libLTO interface, you can stop reading now.

As of r77960 I've removed the lto_codegen_set_gcc_path call from libLTO. This wasn't in the last LLVM release and has already been obsoleted by lto_codegen_set_assembler_path. This shouldn't be a problem for any libLTO users that I know of, but if you were using this, and you can't switch to set_assembler_path, please reply and explain your situation.