The dieFromSignal runtime function attempts to forward crashing
signals to a signal handler registered before the runtime was
initialized, if any. However, on Darwin, a special signal handler
trampoline is invoked, even for non-Go signal handlers.

Clear the crashing signal's handlingSig entry to ensure sigtramp
forwards the signal.

Fixes the darwin/386 builder.

Updates #20392
Updates #19389

Change-Id: I441a3d30c672cdb21ed6d8f1e1322d7c0e5b9669
Reviewed-on: https://go-review.googlesource.com/55032
Run-TryBot: Elias Naur <elias.naur@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>