Fix build error in cross-build with presym

Fix the following issues.

* https://github.com/mruby/mruby/pull/5220#issuecomment-757204258
* https://github.com/mruby/mruby/pull/5220#issuecomment-757205572

lib/mruby/build.rb

@@ -515,11 +515,7 @@ EOS
       unless mrbcfile_external? || MRuby.targets['host']
         # add minimal 'host'
         MRuby::Build.new('host') do |conf|
-          if ENV['VisualStudioVersion'] || ENV['VSINSTALLDIR']
-            toolchain :visualcpp
-          else
-            toolchain :gcc
-          end
+          conf.toolchain
           conf.build_mrbc_exec
           conf.disable_libmruby
           conf.disable_presym
@@ -527,6 +523,10 @@ EOS
       end
     end
 
+    def mrbcfile
+      mrbcfile_external? ? super : MRuby::targets['host'].mrbcfile
+    end
+
     def run_test
       @test_runner.runner_options << verbose_flag
       mrbtest = exefile("#{build_dir}/bin/mrbtest")