Add gem.deps.rb platforms filtering skeleton

This provides only the most basic support.  Version and multiple
platforms are not supported.
Handle multiple platforms in gem.deps.rb

Versions are not checked yet.
Reset Gem.ruby_version for tests

This fixes test bugs where the incorrect ruby version would be chosen.
Reset Gem.ruby_version for tests

If the tests change the RUBY_VERSION a mismatch will occur which may
lead to test failures.
Add gem.deps.rb platform version filtering

The only piece remaining appears to be ruby engine filtering.
Raise for unknown gem.deps.rb platforms

This was originally implemented for the VERSION_MAP, but the
PLATFORM_MAP should have it too.