ruby.c: gem_prelude to load rubygems

* ruby.c (process_options): use gem_prelude instead of requiring
  rubygems directly when --enable=gems is given.
* Makefile.in (DEFAULT_PRELUDES): always use gem_prelude regardless of
  --disable-rubygems.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43278 b2dd03c8-39d4-4d8f-98ff-823fe69b080e