process.c: disable vfork on OS X

* process.c (retry_fork_async_signal_safe): revert r47439 but
  disable use of vfork on OS X instead, as it cause hang-up at
  test_process.rb:test_deadlock_by_signal_at_forking with parallel
  test.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47441 b2dd03c8-39d4-4d8f-98ff-823fe69b080e