libmruby stub

For example, hope to make minimum executable, or shared library using mruby but not using gems, it should be link external libraries to resolve needless linkages. If linking only libmruby_core.a, that need entry point of `mrb_init_mrblib` and `mrb_init_mrbgems`. Of course it should be stub code.

So I hope to add libmruby_stub.a to make minimum executable or shared libraries.