Make it possible that `libmruby.a` is not created

Previously, `libmruby.a` was created even if only `mruby-bin-mrbc` or`
mruby-compiler` was specified for gem, but by specifying `disable_libmruby`,
the creation of `libmruby.a` can be suppressed.

### Note

The https://github.com/mruby/mruby/pull/5084#issuecomment-723521971
incompatibility seems to be difficult for users to avoid, so the original
behavior has been restored. Therefore, if we need `mrbc` other than the
`host` build, we need to explicitly specify `mruby-bin-mrbc` gem.

Due to the above changes, `build_config/boxing.rb` etc. will not work, but I
have added` mruby-bin-mrbc` to `default.gembox` to fix it. I don't think
this change is a big deal because originally `mruby-compiler` was included.

1 files changed, 1 insertions, 5 deletions

diff --git a/tasks/core.rake b/tasks/core.rake
index c4816d4ce..aca5faed8 100644
--- a/tasks/core.rake
+++ b/tasks/core.rake
@@ -9,9 +9,5 @@ MRuby.each_target do
       objfile(dst)
     end
   end
-  self.libmruby_objs << objs
-
-  file libmruby_core_static => objs do |t|
-    archiver.run t.name, t.prerequisites
-  end
+  self.libmruby_core_objs << objs
 end