build: Add variable printing target to Makefiles

Summary:

I kept finding myself needing these to debug our build system, since
they are innocuous and are very helpful they probably belong in the
codebase.

Backport of core PR17087.

Can be useful for the depends system.

Test Plan:

cd depends
make print-build_arch

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5652