gdb: make-target-delegates.py: add Entry type

Add the Entry type and use it in the `entries` map, rather than using an
ad-hoc str -> str map that comes from the re.match.  This will make it
easier to make typing work in a subsequent patch, but it also helps
readers know what attributes exist for entries, which is not clear
currently.

Change-Id: I5b58dee1ed7ae85987b99bd417e641ede718624c
Reviewed-By: Tom Tromey <tom@tromey.com>
Reviewed-By: Andrew Burgess <aburgess@redhat.com>