base: Rework bitunions so they can be more flexible.

base: Rework bitunions so they can be more flexible.

They are now oriented around a class which makes it easy to provide
custom setter/getter functions which let you set or read bits in an
arbitrary way.

Future additions may add the ability to add custom bitfield methods,
and index-able bitfields.

Change-Id: Ibd6d4d9e49107490f6dad30a4379a8c93bda9333
Reviewed-on: https://gem5-review.googlesource.com/7201
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Gabe Black <gabeblack@google.com>