Set `GEM_PATH` environment variable in runruby.rb

tool/runruby.rb

@@ -122,6 +122,12 @@ if e = ENV["RUBYLIB"]
 end
 env["RUBYLIB"] = $:.replace(libs).join(File::PATH_SEPARATOR)
 
+gem_path = [abs_archdir, srcdir].map {|d| File.realdirpath(".bundle", d)}
+if e = ENV["GEM_PATH"]
+ gem_path |= e.split(File::PATH_SEPARATOR)
+end
+env["GEM_PATH"] = gem_path.join(File::PATH_SEPARATOR)
+
 libruby_so = File.join(abs_archdir, config['LIBRUBY_SO'])
 if File.file?(libruby_so)
   if e = config['LIBPATHENV'] and !e.empty?