Prepare stub functions for libmruby_core in core

[dearblue]

The MRuby::Build#libmruby_core_only_objs accessor has been added for this purpose. It probably does not make sense for users to use this accessor.

And, the mruby-bin-mrbc/src/stub.c file has been moved to mruby/src/coreonly/stub.c. The object files generated from this file are passed to the newly created accessor.

[matz]

I agree with the basic idea of this pull-request. But I don't like the directory name coreonly which role is not clear from the directory name.

[dearblue]

Thanks for your comment.
🤔 Hmmm, is src/libmruby_core/ acceptable?

[matz]

src/nogem maybe?

[dearblue]

MRB_NO_GEMS may be defined in libmruby.a builds.

# build configuration
MRuby::Build.new do
  toolchain
end

Also, I don't think libmruby_core.a is very good about nogem since it doesn't include mruby/mrblib.

However, if you think src/nogem is OK, I will update this patch accordingly.