benchmark.rb: Process::CLOCK_MONOTONIC_RAW may be unavailable

* lib/benchmark.rb (BENCHMARK_CLOCK): Process::CLOCK_MONOTONIC_RAW
  is not supported on old linux.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47330 b2dd03c8-39d4-4d8f-98ff-823fe69b080e