use mrbc of current build in non cross build

tasks/mruby_build.rake

@@ -103,7 +103,7 @@ module MRuby
     end
 
     def mrbcfile
-      MRuby.targets['host'].exefile("#{MRuby.targets['host'].build_dir}/bin/mrbc")
+      MRuby.targets[@name].exefile("#{MRuby.targets[@name].build_dir}/bin/mrbc")
     end
 
     def compilers
@@ -204,6 +204,10 @@ module MRuby
   super
     end
 
+    def mrbcfile
+      MRuby.targets['host'].exefile("#{MRuby.targets['host'].build_dir}/bin/mrbc")
+    end
+
     def run_test
       mrbtest = exefile("#{build_dir}/test/mrbtest")
       if (@test_runner.command == nil)