[openmp] [test] Fix warnings about printf format mismatches on Windows

This fixes warnings like this:

```
openmp/runtime/test/omp_testsuite.h:107:60: warning: format specifies type 'unsigned int' but the argument has type 'DWORD' (aka 'unsigned long') [-Wformat]
    fprintf(stderr, "CreateThread() failed: Error #%u.\n", GetLastError());
                                                   ~~      ^~~~~~~~~~~~~~
                                                   %lu
```

Differential Revision: https://reviews.llvm.org/D137747