Test builds: new user_mutex code

[cocobean]

For hrev58089 x64:

• See: #11815 (pthread Posix test suite failures) – Haiku (haiku-os.org)

  • pthread_cancel (test 5-1) - FAILED
  • pthread_detach (test 4-1, 4-2) - FAILED
  • pthread_mutex_lock (test 3-1) - triggers Debugger

I’ll review patches…