If two goroutines are racing on a map, one of them will exit
cleanly, clearing the hashWriting bit, and the other will
likely notice and panic.  If we use XOR instead of OR to
set the bit in the first place, even numbers of racers will
hopefully all see the bit cleared and panic simultaneously,
giving the full set of available stacks.  If a third racer
sneaks in, we are no worse than the current code, and
the generated code should be no more expensive.

In practice, this catches most racing goroutines even in
very tight races.  See the demonstration program posted
on https://github.com/golang/go/issues/26703 for an example.

Fixes #26703

Change-Id: Idad17841a3127c24bd0a659b754734f70e307434
Reviewed-on: https://go-review.googlesource.com/126936
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>