cmd/nm: fix TestGoExec on Plan 9

CL 115975 changed TestGoExec to check symbol types.
However, this test is failing on Plan 9, because
there is no read-only data segment symbol on Plan 9.

This change fixes TestGoExec to replace the check
of read-only data segment symbol (R) by data segment
symbol (D) on Plan 9.

Fixes #25820.

Change-Id: I7164cd9056fa1dfcd1dc1b0f87653290c14c85fa
Reviewed-on: https://go-review.googlesource.com/118035


Run-TryBot: David du Colombier <0intro@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>