"sigaltstack too small in native thread" in detach_signal test

This happened once on the aarch64-sve-precommit-256 test:

https://github.com/DynamoRIO/dynamorio/actions/runs/9914012718/job/27392240520?pr=6879
```
2024-07-12T21:08:37.5756527Z 358: detaching
2024-07-12T21:08:37.5756915Z 358: signal count post-detach: 66757
2024-07-12T21:08:37.5757319Z 358: native signals delivered: 59542
2024-07-12T21:08:37.5758963Z 358:
```

This code was recently changed by #6815 so it is possible this is an introduced regression.