FEATURE: allow specifying platform name for platform specific gems (#10559)

lib/plugin_gem.rb

@@ -6,7 +6,9 @@ module PluginGem
 
     gems_path = File.dirname(path) + "/gems/#{RUBY_VERSION}"
     spec_path = gems_path + "/specifications"
-    spec_file = spec_path + "/#{name}-#{version}.gemspec"
+    spec_file = spec_path + "/#{name}-#{version}"
+    spec_file += "-#{opts[:platform]}" if opts[:platform]
+    spec_file += ".gemspec"
     unless File.exists? spec_file
       command = "gem install #{name} -v #{version} -i #{gems_path} --no-document --ignore-dependencies --no-user-install"
       if opts[:source]