gdb: add git trailer information on gdb/MAINTAINERS

gdb: add git trailer information on gdb/MAINTAINERS

The project has been using Tested-By (tb), Reviewed-By (rb) and
Approved-By (ab) for some time, but there has been no information to be
found in the actual repository. This commit changes that by adding
information about all git trailers to the MAINTAINERS file, so that it
can be easily double-checked. Simply put, the trailers in use work as
follows:

* Tested-by: The person tested the patch and it fixes the problem, or
  introduces no regressions (or both).
* Acked-by: The general outline looks good, but the maintainer hasn't
  looked at the code
* Reviewed-by: The code looks good, but the reviewer has not approved
  the patch to go upstream
* Approved-by: The patch is ready to be pushed to master

These last 3 trailers can also be restricted to one or more areas of GDB
by adding the areas in a comma separated list in parenthesis after the
trailers.

Finally, for completeness sake, the trailers Co-Authored-By and Bug
were added, even though they have been in use for a long time already

Reviewed-By: Kevin Buettner <kevinb@redhat.com>
Reviewed-By: Luis Machado <luis.machado@arm.com>
Approved-By: John Baldwin <jhb@FreeBSD.org>