cmd/go: fix -n output in runtime/internal/atomic

When building runtime/internal/atomic, the toolchain writes a symabis2
file. This file is read back in, filtered, and appended to the symabis
file. This breaks with -n, since the symabis2 file is never written.

With this change, when -n is used, an equivalent "grep" command is
printed instead. The output for -x is unchanged.

Fixes #29346

Change-Id: Id25e06e06364fc6689e71660d000f09c649c4f0c
Reviewed-on: https://go-review.googlesource.com/c/155480


Reviewed-by: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>