[onnxruntime_perf_test] Fix Mac peak working set size value (#27302)

### Description
<!-- Describe your changes. -->

On Mac, `ru_maxrss` is in bytes and on other platforms, it is in
kilobytes. This change sets a different multiplier value when
`defined(__APPLE__)`.

### Motivation and Context
<!-- - Why is this change required? What problem does it solve?
- If it fixes an open issue, please link to the issue here. -->

Fix peak working set size reporting on Mac.

Author: edgchen1

onnxruntime/test/perftest/posix/utils.cc

@@ -17,7 +17,14 @@ namespace utils {
 std::size_t GetPeakWorkingSetSize() {
   struct rusage rusage;
   getrusage(RUSAGE_SELF, &rusage);
-  return static_cast<size_t>(rusage.ru_maxrss * 1024L);
+
+#if defined(__APPLE__)
+  constexpr size_t kBytesPerMaxRssUnit = 1;
+#else
+  constexpr size_t kBytesPerMaxRssUnit = 1024;
+#endif
+
+  return static_cast<size_t>(rusage.ru_maxrss) * kBytesPerMaxRssUnit;
 }
 
 class CPUUsage : public ICPUUsage {