base: Enable specializing templates on BitUnion types.

base: Enable specializing templates on BitUnion types.

Previously these relied on reaching into private internal definitions
in the BitUnion types.

Change-Id: Ia6c94de92986b85ec9e5fcb197459d450111fb36
Reviewed-on: https://gem5-review.googlesource.com/7202
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Gabe Black <gabeblack@google.com>