runtime: fix runtime-gdb script loading

runtime.rtype was a copy of reflect.rtype - update script to use that directly.
Introduces a basic test which will skip on systems without appropriate GDB.

Fixes #9326

Change-Id: I6ec74e947bd2e1295492ca34b3a8c1b49315a8cb
Reviewed-on: https://go-review.googlesource.com/2821


Reviewed-by: Ian Lance Taylor <iant@golang.org>