Add support for the x64-mingw32 platform

Bundler currently does not handle (binary) gems for the x64-mingw32 platform correctly. This patch solves this issue by recognizing x64-mingw32 as a valid platform and by adding :x64_mingw as a new platform tag. Hereby :platform=>:mingw is completely distinct from :platform=>:x64_mingw.

This patch also solves issue #2356.