On some systems, gdb is set to: "startup-with-shell on". This
breaks runtime_test. This just make sure gdb does not start by
spawning a shell.

Fixes #15354

Change-Id: Ia040931c61dea22f4fdd79665ab9f84835ecaa70
Reviewed-on: https://go-review.googlesource.com/23142Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>