Define NATIVE_RSTRING_PTR in truffleruby.h
Define NATIVE_RSTRING_PTR in truffleruby.h

* So we can reliably force the RSTRING_PTR to go native.
* Add TruffleRuby spec to verify that it works as intended.

ruby/spec,ruby/spec,eregon/rubyspec,eregon/rubyspec,nobu/rubyspec,nobu/rubyspec,eregon/rubyspec,nobu/rubyspec,ruby/spec