cmd/go: look for __go_buildinfo section when looking for Mach-O version

This mirrors the ELF fix in CL 188957. TestScript/version failed on
darwin after that change.

Fixes #31861

Change-Id: I4ce953ebec8dd5fa47e26d373c59d7e290b75a34
Reviewed-on: https://go-review.googlesource.com/c/go/+/189159


Run-TryBot: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>