support for GEM_HOME_SHARED / GEM_HOME_PLATFORM

would it be possible to support second `GEM_HOME` like `GEM_HOME_PLATFORM` which would be used for gems that use in specification:

- [required_ruby_version](http://docs.rubygems.org/read/chapter/20#required_ruby_version)
- [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` for all rubies (or even unset it) and only manipulate `GEM_HOME_PLATFORM` when changing rubies

or to make the migration easier use `GEM_HOME` for platform specific gems and introduce `GEM_HOME_SHARED` which will host all gems that are not platform specific - this way all tools that use already `GEM_HOME` per ruby version/implementation would still work as expected and with simple `export GEM_HOME_SHARED=...` any gems that are not platform specific would be shared across all rubies.