Extract a helper function, linksymname.
This simplifies Linksym,
and linksymname will be useful in future work.

Change-Id: Ic5ff8b704a16d5020f6931e008e2f630f687cbd3
Reviewed-on: https://go-review.googlesource.com/40550
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>