GNU libgomp: Implementing PARALLEL construct

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### 9.10 Implementing PARALLEL construct

      #pragma omp parallel
      {
        body;
      }

becomes

      void subfunction (void *data)
      {
        use data;
        body;
      }

      setup data;
      GOMP_parallel_start (subfunction, &data, num_threads);
      subfunction (&data);
      GOMP_parallel_end ();

      void GOMP_parallel_start (void (*fn)(void *), void *data, unsigned num_threads)

The FN argument is the subfunction to be run in parallel.

The DATA argument is a pointer to a structure used to communicate data in and out of the subfunction, as discussed above with respect to FIRSTPRIVATE et al.

The NUM_THREADS argument is 1 if an IF clause is present and false, or the value of the NUM_THREADS clause, if present, or 0.

The function needs to create the appropriate number of threads and/or launch them from the dock. It needs to create the team structure and assign team ids.

      void GOMP_parallel_end (void)

Tears down the team and returns us to the previous `omp_in_parallel()` state.