cmd/link: fix bug in -strictdups checking of BSS symbols

The linker's -strictdups debugging option was not properly checking
for cases where you have two dupok BSS symbols with different length
(the check examined data length and content, but not symbol size).

Updates #46653.

Change-Id: I3844f25ef76dd6e4a84ffd5caed5d19a1b1a57c9
Reviewed-on: https://go-review.googlesource.com/c/go/+/326210


Trust: Than McIntosh <thanm@google.com>
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>