I was about to write off this benchmark while working on GVL
improvements on multi-core systems.

However I noticed it exposes a weakness in my work-in-progress
code when I tested on an old single CPU system.  Further testing
reveals setting CPU affinity ("schedtool -a 0x1" on Linux) on a
modern multi-core system is enough to reproduce the problem
exposed by this benchmark.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@58571 b2dd03c8-39d4-4d8f-98ff-823fe69b080e