Some cleanup to common.gypi. This fixes some host/target combinations that weren't working in the Make build on Mac.

Review URL: https://chromiumcodereview.appspot.com/10416005

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@11617 ce2b1a6d-e550-0410-aec6-3dcde31c8c00