[OpenMP] libomp: change shm name to include UID, call unregister_lib on SIGTERM

With the change to using shared memory, there were a few problems that need to be fixed.
- The previous filename that was used for SHM only used process id. Given that process is
  usually based on 16bit number, this was causing some conflicts on machines. Thus we add
  UID to the name to prevent this.
- It appears under some conditions (SIGTERM, etc) the shared memory files were not getting
  cleaned up. Added a call to clean up the shm files under those conditions. For this user
  needs to set envirable KMP_HANDLE_SIGNALS to true.

Patch by Erdner, Todd <todd.erdner@intel.com>

Differential Revision: https://reviews.llvm.org/D91869