Add serenity-32bit crossbuild

Cross compile mruby for SerenityOS (https://github.com/SerenityOS/serenity)

build_config/serenity-32bit.rb

@@ -0,0 +1,12 @@
+MRuby::CrossBuild.new('serenity-32bit') do |conf|
+  conf.toolchain :gcc
+
+  conf.archiver.command = 'i686-pc-serenity-ar'
+  conf.linker.command = 'i686-pc-serenity-g++'
+  conf.cc.command = 'i686-pc-serenity-gcc'
+  conf.cc.defines << 'MRB_32BIT'
+
+  conf.gembox 'full-core'
+
+  conf.test_runner.command = 'env'
+end