scons: Several fixes having to do with tags and sets.

scons: Several fixes having to do with tags and sets.

There were a few places where tags weren't being converted to sets
correctly which unfortunately only manifested when called in certain
ways. This would be a pretty reasonable place to add some python unit
tests...

Change-Id: I87509369b4ec6f702b7521e52bf63701a87ec436
Reviewed-on: https://gem5-review.googlesource.com/6261
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>