cmd/go: fix reading PT_NOTE segment with multiple notes

cmd/go: fix reading PT_NOTE segment with multiple notes

The old code was assuming that a PT_NOTE segment never had more than one
note, but there is no such requirement.

Fixes #13364.

Change-Id: I3f6b3716130bf7af6abe81b8e10571a8c7cd943c
Reviewed-on: https://go-review.googlesource.com/17331
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>