Implementing MASKED and MASTER construct (GNU libgomp)

### 13.1 Implementing MASKED and MASTER construct ¶

    if (GOMP_has_masked_thread_num(thread_num))
      block

`GOMP_has_masked_thread_num` encapsulates the following check, so that OMPT can distinguish the compiler-generated code from user-level thread-number queries:

    omp_get_thread_num() == thread_num

Hereby, thread_num has the value of the argument to the `filter` clause or zero if not specified.

Alternately, we generate two copies of the parallel subfunction and only include this in the version run by the thread_num thread. Surely this is not worthwhile though...