runtime: make runtime-gdb.py tolerant of creatively-named gdb versions

runtime: make runtime-gdb.py tolerant of creatively-named gdb versions

"Fedora" and "Red Hat" are not numbers, it turns out.
Don't rely on version numbers, instead use a regexp to
handle variation across the 2 patterns thus far observed
for gdb-generated Go type names.

Change-Id: I18c81aa2848265a47daf1180d8f6678566ae3f19
Reviewed-on: https://go-review.googlesource.com/c/go/+/236280
Reviewed-by: Austin Clements <austin@google.com>
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>