macOS and iOS deliver SIGPIPE signals to the main thread and not
the thread that raised it by writing to a closed socket or pipe.

SIGPIPE signals can be suppressed for sockets with the SO_NOSIGPIPE
option, but there is no similar option for pipes. We have no other
choice but to never forward SIGPIPE on macOS.

This is a fixup of reverted CL 188297.

Fixes #33384

Change-Id: I09b258b078857ad3b22025bc2902d1b12d2afd92
Reviewed-on: https://go-review.googlesource.com/c/go/+/191785
Run-TryBot: Elias Naur <mail@eliasnaur.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>