[CMAKE] Generate the split-debug.sh script

Summary:
The split-debug.sh script is generated from a template at build time,
and used by the linux gitian build to separate the debug symbols from
the stripped binaries.

Test Plan:
For all the linux platforms:

cmake ..

Check the <build_dir>/contrib/devtools/split-debug.sh script is
generated as expected.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4350