Add new two build config files.

examples/targets/build_config_android_arm64-v8a.rb

+MRuby::Build.new do |conf|
+
+  # Gets set by the VS command prompts.
+  if ENV['VisualStudioVersion'] || ENV['VSINSTALLDIR']
+    toolchain :visualcpp
+  else
+    toolchain :gcc
+  end
+
+  enable_debug
+
+  # include the default GEMs
+  conf.gembox 'default'
+end
+
+# Requires Android NDK r13 or later.
+MRuby::CrossBuild.new('android-arm64-v8a') do |conf|
+  params = { 
+    :arch => 'arm64-v8a', 
+    :platform => 'android-24',
+    :toolchain => :clang,
+  }
+  toolchain :android, params
+
+  conf.gembox 'default'
+end

examples/targets/build_config_android_armeabi.rb

+MRuby::Build.new do |conf|
+
+  # Gets set by the VS command prompts.
+  if ENV['VisualStudioVersion'] || ENV['VSINSTALLDIR']
+    toolchain :visualcpp
+  else
+    toolchain :gcc
+  end
+
+  enable_debug
+
+  # include the default GEMs
+  conf.gembox 'default'
+end
+
+# Requires Android NDK r13 or later.
+MRuby::CrossBuild.new('android-armeabi') do |conf|
+  params = { 
+    :arch => 'armeabi', 
+    :platform => 'android-24',
+    :toolchain => :clang,
+  }
+  toolchain :android, params
+
+  conf.gembox 'default'
+end