HANG drcachesim online across setarch execve

This surprised me b/c I thought we had multi-process tests including execve, so there is likely something particular about this specific execve?

I just wanted to disable ASLR for the app and it hangs:

```
$ bin64/drrun -t drcachesim -- setarch `uname -m` -R suite/tests/bin/simple_app

<(1+x) Handling our fault in a TRY at 0x00007f8597a8d93b>
<-- execve suite/tests/bin/simple_app -->

<(1+x) Handling our fault in a TRY at 0x00007ffff7e1c93b>
```

It just hangs there trying to read the pipe. So sthg with the pipe is not handled properly across that execve it seems. Not sure how other tests w/ execve differ.

In any case the workaround here is to run `drrun` under the `setarch`.