runtime: populate signal PC on NetBSD

The NetBSD libc provides an architecture-independent macro that can
extract the PC from a ucontext struct.

Change-Id: I94534ee32d54bae4ead5cafc964bdace058a4322
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/261740
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Trust: Than McIntosh <thanm@google.com>
1 file changed