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>

src/python/importer.py

@@ -77,7 +77,7 @@ class CodeImporter(object):
                 mod.__package__ = fullname.rpartition('.')[0]
             mod.__file__ = srcfile
 
-            exec code in mod.__dict__
+            exec(code, mod.__dict__)
         except Exception:
             del sys.modules[fullname]
             raise