gdb: rename target-delegates.c to target-delegates-gen.c

Following this suggestion:

https://inbox.sourceware.org/gdb-patches/2a0520ec-ccfe-4fc3-b051-7b8c60294de5@efficios.com/T/#md537792a1871addf153f3e406224f9baf025414a



Change-Id: I30988c46505f130ca16155891958f92621cada97
Approved-By: John Baldwin <jhb@FreeBSD.org>
Approved-By: Andrew Burgess <aburgess@redhat.com>