debug/gosym: fix TestPCLine on Plan 9

debug/gosym: fix TestPCLine on Plan 9

Plan 9 doesn't define main, so the INITENTRY
symbol remains with the SXREF type, which leads
Entryvalue to fail on "entry not text: main".

Fixes #14536.

Change-Id: Id9b7d61e5c2202aba3ec9cd52f5b56e0a38f7c47
Reviewed-on: https://go-review.googlesource.com/19973
Run-TryBot: David du Colombier <0intro@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>