sparc: Pull StaticInst base classes out of the ISA description.

Also, do some minor refactoring to use a BitUnion to pull apart
condition codes, etc.

Change-Id: I0c88878b07a731d0c0fe30f264f53dd795db99ae
Reviewed-on: https://gem5-review.googlesource.com/5421
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Gabe Black <gabeblack@google.com>