benchmark.rb: prefer Process::CLOCK_MONOTONIC_RAW

* lib/benchmark.rb (BENCHMARK_CLOCK): prefer Process::CLOCK_MONOTONIC_RAW
  if available to more accurate measure.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47320 b2dd03c8-39d4-4d8f-98ff-823fe69b080e