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

build_config/cross-32bit.rb

 # Define cross build settings
 MRuby::CrossBuild.new('cross-32bit') do |conf|
-  toolchain :gcc
+  conf.toolchain :gcc
 
   conf.cc.flags << "-m32"
   conf.linker.flags << "-m32"

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")

tasks/presym.rake

+mrbtest = "#{MRuby::Build.install_dir}/mrbtest"
 all_prerequisites = ->(task_name, prereqs) do
   Rake::Task[task_name].prerequisites.each do |prereq_name|
     next if prereqs[prereq_name]
@@ -18,13 +19,14 @@ MRuby.each_target do |build|
 
   prereqs = {}
   pps = []
-  mrbtest = "#{build.class.install_dir}/mrbtest"
+  build_dir = "#{build.build_dir}/"
   mrbc_build_dir = "#{build.mrbc_build.build_dir}/" if build.mrbc_build
   build.products.each do |product|
     all_prerequisites.(product, prereqs) unless product == mrbtest
   end
   prereqs.each_key do |prereq|
     next unless File.extname(prereq) == build.exts.object
+    next unless prereq.start_with?(build_dir)
     next if mrbc_build_dir && prereq.start_with?(mrbc_build_dir)
     pps << prereq.ext(build.exts.preprocessed)
   end