cmd/link: fix off-by-1 error in findShlibSection

We want to find a section that contains addr. sect.Addr+sect.Size
is the exclusive upper bound.

Change-Id: If2cd6bdd6e03174680e066189b0f4bf9e2ba6630
Reviewed-on: https://go-review.googlesource.com/c/go/+/288592


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