[release-branch.go1.17] cmd/link: disable weak reference in itab if build with "-linkshared"

When build with "-linkshared", we can't tell if the interface method will be used or not. It can be used in shared library.
This CL backport this fix to 1.17.

Fixes #49086

Change-Id: Iba12812f199b7679cf2fd41a304268d6d6dd03c6
Reviewed-on: https://go-review.googlesource.com/c/go/+/350189


Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Alexander Rakoczy <alex@golang.org>
(cherry picked from commit f687831e)
Reviewed-on: https://go-review.googlesource.com/c/go/+/357231
Trust: Ian Lance Taylor <iant@golang.org>