runtime: fix prettyprinting of parametric types in gdb

golang.org/cl/344929 broke the minimal functionality that the python
pretty printer for GDB had, this change restores it to its status prior
to that CL.

Change-Id: I4c7141d4ff726d224a074ecc533d0f896fc0052c
Reviewed-on: https://go-review.googlesource.com/c/go/+/350529


TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Trust: Than McIntosh <thanm@google.com>