rake/cpu_counter.rb: use Etc.nprocessors

* lib/rake/cpu_counter.rb (count): prefer Etc.nprocessors, which
  is hundreds times faster.

* test/rake/test_rake_cpu_counter.rb: add tests for features, and
  remove useless tests.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48127 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2014-10-25

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48128 b2dd03c8-39d4-4d8f-98ff-823fe69b080e