[libomptarget][test] add return 0 to map_back_race.cpp

[ronlieb]

Summary

sporadic failrues in amdgpu buildbot for the test map_back_race.cpp
seems to be missing return 0; in function main.

[saiislam]

LGTM

[jdoerfert]

No objects, but this is pointless, IMHO.

https://reviews.llvm.org/D121058#3413874

[ronlieb]

having looked at the generated code for g++ and clang++ , i agree this does not and will not fix the underlying issue.
abandoning

openmp/libomptarget/test/mapping/map_back_race.cpp

#pragma omp atomic update
     }
   }
   if (!almost_equal(counter_N0, expected_value, 0.01)) {
     std::cerr << "Expected: " << expected_value << " Got: " << counter_N0
               << std::endl;
     std::exit(112);
   }
 }
-int main() { test_parallel_for__target(); }
+int main() {
+  test_parallel_for__target();
+  // above function returns no status
+  return 0;
+}