tool.drcachesim.multiproc failed on ARM in translate_sigcontext

Testing on some new hardware, and possibly a different version of Ubuntu, we got this, which we hadn't seen before:

```
105: Test command: /.../dynamorio_build/bin32/drrun "-s" "90" "-quiet" "-debug" "-killpg" "-stderr_mask" "0xC" "-dumpcore_mask" "0" "-code_api" "-t" "drcachesim" "-ipc_name" "drtestpipe3" "--" "/.../dynamorio_build/suite/tests/bin/tool.multiproc" "/.../dynamorio_build/suite/tests/bin/tool.multiproc"
105: Test timeout computed to be: 1500
105:
105: all done
105: ---- ----
```

The corresponding source code (signal.c:2189) is:

```
if (avoid_failure) {
ASSERT_NOT_REACHED(); /* is ok to break things, is UNIX :) */
/* FIXME : what to do? reg state might be wrong at least get pc */
```