[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.