Rewrite make-target-delegates in Python

Rewrite make-target-delegates in Python

I think gdb is probably better off having fewer languages involved
when generating code.  'sh' is unavoidable for build-time generation,
but for other things, let's use Python.

This rewrites make-target-delegates in Python.  I've stuck pretty
closely to the original code in this rewrite, so it may look slightly
weird from a Python perspective.

The only output difference is that a copyright header is now
generated, using the code introduced in the previous patch.

make-target-delegates.py is simpler to invoke, as it knows the correct
input file to scan and it creates the output file itself.