runtime: check gdb exit status and log output

All GDB tests currently ignore non-zero exit statuses. When tests
flakes, we don't even know if GDB exited successfully or not.

Add checks for non-zero exits, which are not expected.

Furthermore, always log the output from GDB. The tests are currently
inconsistent about whether they always log, or only on error.

Updates #39021

Change-Id: I7af1d795fc2fdf58093cb2731d616d4aa44e9996
Reviewed-on: https://go-review.googlesource.com/c/go/+/235282


Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>