lib/benchmark.rb: speedup by reducing allocations

* lib/benchmark.rb (module Benchmark): define BENCHMARK_CLOCK
  (realtime): use Process.clock_gettime(BENCHMARK_CLOCK)
  [Feature #10165]

* test/benchmark/test_benchmark.rb (test_realtime_output): new test

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47260 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2014-08-24

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47261 b2dd03c8-39d4-4d8f-98ff-823fe69b080e