* benchmark/gc: create a directory to store GC related benchmark.
* benchmark/gc/gcbench.rb: moved from tool/gcbench.rb.
* benchmark/gc/hash(1|2).rb: ditto.
* benchmark/gc/rdoc.rb: ditto.
* benchmark/gc/null.rb: added.
* common.mk: fix rule.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41473 b2dd03c8-39d4-4d8f-98ff-823fe69b080e