supported by legacy benchmark/driver.rb.

benchmark/README.md: document them

common.mk: update benchmark_driver to correct 0.0 output and to fix
spacing format of `-o simple` and `-o markdown`.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@63933 b2dd03c8-39d4-4d8f-98ff-823fe69b080e