runtime: allow OutputDebugString to be sent to debugger

We mark DBG_PRINTEXCEPTION_C messages in VEH handler
as handled, thus preventing debugger from seeing them.
I don't see reason for doing that. The comment warns
of crashes, but I added test and don't see any crashes.

This is also simplify VEH handler before making
changes to fix issue 8006.

Update issue 8006

LGTM=rsc
R=golang-codereviews, rsc
CC=golang-codereviews
https://codereview.appspot.com/146800043
runtime: revise TestSetPanicOnFault

We can't assume all those addresses are unmapped.
But at least one should be.

What we're really testing is that the program doesn't crash.

Fixes issue 8542.

LGTM=iant
R=golang-codereviews, iant, minux
CC=golang-codereviews
https://codereview.appspot.com/144120043