[CI] Add cross builds configuration

Summary:
This diff adds a cross_build configuration element which allow for
setting the static dependencies target, the toolchain file and
optionally an emulator.

Currently only the linux cross builds are convertd, because windows and
osx needs some other changes (respectively filtering the tests and
overriding the python path);

Test Plan: Run the linux-* CI builds.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7380