cpu_counter.rb: separate implementations

* lib/rake/cpu_counter.rb (Rake::CpuCounter#count): separate
  Etc.nprocessors implementation, and if is is available other
  utility methods are never used.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48279 b2dd03c8-39d4-4d8f-98ff-823fe69b080e