HANG in linux.sigmask-noalarm test

Happened once on 64-bit GA CI:
https://github.com/DynamoRIO/dynamorio/runs/8241241189?check_suite_focus=true
```
155/431 Test #91: code_api|linux.sigmask-noalarm ...............................***Failed Required regular expression not found. Regex=[^sending 10
in handler for signal 10
sending 28 with value
in handler for signal 28 from -1 value 0xdeadbeef
in handler for signal 12
init thread now inside handler: setting up itimer
done with itimer; exiting
all done
$
] 90.00 sec
sending 10
in handler for signal 10
sending 28 with value
in handler for signal 28 from -1 value 0xdeadbeef
in handler for signal 12
init thread now inside handler: setting up itimer
done with itimer; exiting
```