runtime: fix GDB tests on s390x running Ubuntu 18.04

On Ubuntu 18.04 I am seeing GDB fail to restore the stack pointer
during this test because stack unwinding can't find the PC. This CL
is essentially a partial revert of CL 23940 and fixes the issue on
s390x.

Fixes #33757

Change-Id: Ib4c41162dc85dc882eb6e248330f4082c3fa94c3
Reviewed-on: https://go-review.googlesource.com/c/go/+/169857


Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
(cherry picked from commit d91f7e66)
Reviewed-on: https://go-review.googlesource.com/c/go/+/200039


Run-TryBot: Katie Hockman <katie@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Michael Munday <mike.munday@ibm.com>