[dev.link] cmd/link: create symbol updated lazily in amd64 adddynrel

[dev.link] cmd/link: create symbol updated lazily in amd64 adddynrel

Tweak the code in the amd64 version of adddynrel to avoid creating a
symbol updated for the symbol being processed until it's clear we need
to alter its relocations. This should help performance for the
PIE+internal linking scenario.

Reviewed-on: https://go-review.googlesource.com/c/go/+/229866
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Change-Id: Id25adfd81a5bbd2dde0f80a83b976397ba6abfb5
Reviewed-on: https://go-review.googlesource.com/c/go/+/230026
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>