runtime: do not generate crash dump on Windows 7

It appears Windows 7 ignores WER_FAULT_REPORTING_NO_UI WerSetFlags
API flag.

And now after CL 307372, runtime will display WER GUI dialogue.

We don't want to introduce random GUI dialogues during Go program
execution. So disable dump crash creation on Windows 7 altogether.

Updates #20498

Change-Id: Ie268a7d4609f8a0eba4fe9ecf250856b0a61b331
Reviewed-on: https://go-review.googlesource.com/c/go/+/360617


Trust: Alex Brainman <alex.brainman@gmail.com>
Run-TryBot: Alex Brainman <alex.brainman@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Patrik Nyblom <pnyb@google.com>