support for GEM_HOME_SHARED

would it be possible to support second `GEM_HOME` like `GEM_HOME_SHARED` which would be used for gems that do not use in specification:

- [required_ruby_version](http://docs.rubygems.org/read/chapter/20#required_ruby_version) (1.8.7+)
- [platform](http://docs.rubygems.org/read/chapter/20#platform)
- [extensions](http://docs.rubygems.org/read/chapter/20#extensions)

this way it would be possible to use single `GEM_HOME_SHARED` for all rubies and only manipulate `GEM_HOME` when changing rubies for gems with extensions or platform specific