[CMAKE] Fix FindBerkeleyDB suffix paths

Summary:
The FindBerkeleyDB module builds a combination of component and version
to search for the associated library, but use the same combination for
the suffix paths. However the suffix paths are always based on the name
db (not db_cxx) and can cause the search to fail on some systems.

Test Plan:

cmake -GNinja ..
ninja

Run the Gitian builds.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5446