[CMAKE] Generate the split-debug.sh script
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.
For all the linux platforms:
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