Removed extra ceremony from running IRB.
Fixed setting bootstrap load path.
Fixed loading rubygems with bootstrap libs.
Fixed starting IRB.
Fixed standard library stubs.

We use actual files so that they will be found from the $LOAD_PATH
if other things (like rubygems) override #require.

This also gives us the ability to provide a meaningful error message
that explains how to install the standard lib gem and what to do in
the confusing situations where a gem is installed but GEM_HOME or
GEM_PATH are messed up.
Removed rubygems require override running specs.