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.

lib/mruby/build.rb

@@ -53,7 +53,7 @@ module MRuby
     include Rake::DSL
     include LoadGems
     attr_accessor :name, :bins, :exts, :file_separator, :build_dir, :gem_clone_dir
-    attr_reader :libmruby_objs, :gems, :toolchains, :gem_dir_to_repo_url
+    attr_reader :libmruby_core_objs, :libmruby_objs, :gems, :toolchains, :gem_dir_to_repo_url
 
     alias libmruby libmruby_objs
 
@@ -66,7 +66,7 @@ module MRuby
     def initialize(name='host', build_dir=nil, &block)
       @name = name.to_s
 
-      unless MRuby.targets[@name]
+      unless current = MRuby.targets[@name]
         if ENV['OS'] == 'Windows_NT'
           @exts = Exts.new('.o', '.exe', '.a')
         else
@@ -91,7 +91,9 @@ module MRuby
 
         @bins = []
         @gems = MRuby::Gem::List.new
-        @libmruby_objs = []
+        @libmruby_core_objs = []
+        @libmruby_objs = [@libmruby_core_objs]
+        @enable_libmruby = true
         @build_mrbtest_lib_only = false
         @cxx_exception_enabled = false
         @cxx_exception_disabled = false
@@ -102,15 +104,22 @@ module MRuby
         @toolchains = []
         @gem_dir_to_repo_url = {}
 
-        MRuby.targets[@name] = self
+        MRuby.targets[@name] = self.class.current = current = self
       end
 
-      current = MRuby.targets[@name]
-      MRuby::Build.current = current
       current.instance_eval(&block)
+      current.build_mrbc_exec if current.libmruby_enabled? && @name == "host"
       current.build_mrbtest if current.test_enabled?
     end
 
+    def libmruby_enabled?
+      @enable_libmruby
+    end
+
+    def disable_libmruby
+      @enable_libmruby = false
+    end
+
     def debug_enabled?
       @enable_debug
     end
@@ -255,11 +264,9 @@ EOS
     def mrbcfile
       return @mrbcfile if @mrbcfile
 
-      unless gems.detect {|v| v.name == 'mruby-bin-mrbc' }
-        build_mrbc_exec
-        gems.detect {|v| v.name == 'mruby-bin-mrbc' }.setup
-      end
-      @mrbcfile = self.exefile("#{self.build_dir}/bin/mrbc")
+      gem_name = "mruby-bin-mrbc"
+      gem = gems[gem_name] || MRuby.targets["host"].gems[gem_name]
+      @mrbcfile = exefile("#{gem.build.build_dir}/bin/mrbc")
     end
 
     def compilers
@@ -392,6 +399,7 @@ EOS
             toolchain :gcc
           end
           conf.gem :core => 'mruby-bin-mrbc'
+          conf.disable_libmruby
         end
       end
       @endian = nil
@@ -400,8 +408,7 @@ EOS
     end
 
     def mrbcfile
-      host = MRuby.targets['host']
-      host.exefile("#{host.build_dir}/bin/mrbc")
+      MRuby.targets['host'].mrbcfile
     end
 
     def run_test

lib/mruby/gem.rb

@@ -353,6 +353,10 @@ module MRuby
         @ary.each(&b)
       end
 
+      def [](name)
+        @ary.detect {|g| g.name == name}
+      end
+
       def <<(gem)
         unless @ary.detect {|g| g.dir == gem.dir }
           @ary << gem