Add `target/host-m32.rb` to compile mruby in 32bit mode.

Tested only on Linux. You need to install 32bit relate libraries,
e.g. libc6-dev-i386.

target/host-m32.rb

+MRuby::Build.new do |conf|
+  # load specific toolchain settings
+  toolchain :gcc
+
+  # include the GEM box
+  conf.gembox 'full-core'
+
+  conf.cc.flags << '-m32'
+  conf.linker.flags << '-m32'
+
+  # Turn on `enable_debug` for better debugging
+  enable_debug
+  conf.enable_test
+end