[release-branch.go1.4] cmd/link: specify correct size for dynamic symbols in 386 elf output

This is a backport of https://golang.org/cl/22912 to the Go 1.4 branch.

Updates #20672

Change-Id: Id71c0c4596a599c5293b013f68894f6ee13e06d6
Reviewed-on: https://go-review.googlesource.com/66070


Reviewed-by: Ian Lance Taylor <iant@golang.org>