debug/buildinfo: implement for Plan 9 a.out
debug/buildinfo: implement for Plan 9 a.out

Plan 9 a.out was not implemented for debug/buildinfo, which
was causing test failures on Plan 9. This adds an implementation,
and causes the tests to pass.

Fixes #53949

Change-Id: I90a307ef9babf8cf381f8746d731cac2206b234a
Reviewed-on: https://go-review.googlesource.com/c/go/+/418014
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@google.com>
Reviewed-by: Benny Siegert <05f4aee36fd25d0de2a07789fd6e73398db492cc@gmail.com>
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@google.com>
Auto-Submit: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@google.com>

golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go