sim: Add support for serializing BitUnionXX
sim: Add support for serializing BitUnionXX

BitUnion instances can normally not be used with the SERIALIZE_SCALAR
and UNSERIALIZE_SCALAR macros due to the way they are converted
between their storage type and their actual type. This changeset adds
a set of parm(In|Out) functions specifically for gem5 bit unions to
work around the issue.
(transplanted from 810f5a48a920cf581d61c55dbd201dd554c27aec)

andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin,andrewfu0325/gem5-aladdin