Add `big_endian` and `little_endian` methods to `CrossBuild`; ref #3922

When your target machine is big endian, specify as following in the
`build_config.rb`:

```ruby
MRuby::CrossBuild.new('32bit') do |conf|
  toolchain :gcc

  conf.big_endian
end
```

lib/mruby/build.rb

@@ -334,6 +334,7 @@ EOS
     attr_accessor :host_target, :build_target
 
     def initialize(name, build_dir=nil, &block)
+      @endian = nil
       @test_runner = Command::CrossTestRunner.new(self)
       super
     end
@@ -351,5 +352,26 @@ EOS
         @test_runner.run(mrbtest)
       end
     end
+
+    def big_endian
+      if @endian
+        puts "Endian has already specified as #{@endian}."
+        return
+      end
+      @endian = :big
+      @mrbc.compile_options += ' -E'
+      compilers.each do |c|
+        c.defines += %w(MRB_ENDIAN_BIG)
+      end
+    end
+
+    def little_endian
+      if @endian
+        puts "Endian has already specified as #{@endian}."
+        return
+      end
+      @endian = :little
+      @mrbc.compile_options += ' -e'
+    end
   end # CrossBuild
 end # MRuby