ASSERT single step execution on UNIX

Duplicating for Unix #2144 see as well #2295

With version 6.2.0-2 of DynamoRio
The latest build does not solve the problem

On Linux, with a 32 bit application, using single step exception with trap flag

It can be run with no client
./drrun -- single-step

The expected output is
```
start of test, count = 0
single step exception
end of test, count = 2
```
Instead we get an application end with an assertion.

The problem seems to come from the single step execution set with the trap flag