* process.c: [DOC] MACH_ABSOLUTE_TIME_CLOCK_MONOTONIC is an
  available emulation for a monotonic clock on Darwin.
  https://developer.apple.com/library/mac/qa/qa1398/_index.html

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42594 b2dd03c8-39d4-4d8f-98ff-823fe69b080e