[CMAKE] Improve the toolchain files

Summary:
This sets a few more variables in the toolchain files, that will allow
us to have a better detection of the target architecture in the cmake
list files.

There is no change in behavior.

Test Plan: Run the gitian builds.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5501