HANG tool.drcachesim.phys-threads once on GA CI

[derekbruening]

The new tool.drcachesim.phys-threads test failed once in a PR with a hang:

https://github.com/DynamoRIO/dynamorio/runs/6716249823?check_suite_focus=true

] 90.00 sec

    -------------------------------------------------------------------
     Performance for solving AX=B Linear Equation using Jacobi method
     Running on DynamoRIO
     Client version (null)
    ...................................................................

     Matrix Size :  64
     Threads     :  4

Stopping at that early point in the output and then timing out sure looks like a hang.

Happened just the once.