python: Update use of exec to work with Python 3

python: Update use of exec to work with Python 3

Python 3 uses 'exec(code, globals)' instead of 'exec code in
globals'. Switch to the newer syntax since it is supported by Python
2.7. Also, move check_tracing out of main to work around a bug in
Python 2.7.

Change-Id: I6d390160f58783e1b038a572b64cdf3ff09535fa
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/15986
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>