[LibFuzzer] Fix implementation of ``GetPeakRSSMb()`` on Mac OSX

[delcypher]

Summary

[LibFuzzer]
Fix implementation of `GetPeakRSSMb()` on Mac OSX. On Linux
`rusage.ru_maxrss` is in KiB but on Mac OSX it is in
bytes.

[kcc]

same here, avoid C preprocessor at all costs, it is evil

[delcypher]

Remove use of #ifdefs

[kcc]

LGTM

llvm/trunk/lib/Fuzzer/FuzzerUtil.cpp

std::string Base64(const Unit &U) {
   }
   return Res;
 }
 
 size_t GetPeakRSSMb() {
   struct rusage usage;
   if (getrusage(RUSAGE_SELF, &usage))
     return 0;
+  if (LIBFUZZER_LINUX) {
+    // ru_maxrss is in KiB
     return usage.ru_maxrss >> 10;
+  } else if (LIBFUZZER_APPLE) {
+    // ru_maxrss is in bytes
+    return usage.ru_maxrss >> 20;
+  }
+  assert(0 && "GetPeakRSSMb() is not implemented for your platform");
+  return 0;
 }
 
 }  // namespace fuzzer