(PUP-12031) Add Rake task for building puppet gems

This commit adds a Rake task, gem:build, that will build all puppet
gems which includes the three platform specific puppet gems:
x86-mingw32, x64-mingw32, and universal-darwin

Author: AriaXLi

Diff for: Rakefile

@@ -45,6 +45,16 @@ task :default do
   sh %{rake -T}
 end
 
+namespace :gem do
+  desc 'build puppet gems'
+  task :build do
+    sh "gem build puppet.gemspec --platform x86-mingw32"
+    sh "gem build puppet.gemspec --platform x64-mingw32"
+    sh "gem build puppet.gemspec --platform universal-darwin"
+    sh "gem build puppet.gemspec"
+  end
+end
+
 task :spec do
   ENV["LOG_SPEC_ORDER"] = "true"
   sh %{rspec #{ENV['TEST'] || ENV['TESTS'] || 'spec'}}