runtime: support capturing C backtrace from signal handler on darwin/amd64
The implementation is mostly copied from the commit that added
linux/amd64 support for this feature (https://golang.org/cl/17761).
Change-Id: I3f482167620a7a3daf50a48087f8849a30d713bd
Reviewed-on: https://go-review.googlesource.com/102438 Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>