cmd/link: clean up pclntab
cmd/link: clean up pclntab

More minor cleanup:

* Code simplification
* Move variable declaration closer to use
* Add docs
* Refactor loop

Change-Id: I6f662cb65038b6ad927eb83757b241ac1ef58943
Reviewed-on: https://go-review.googlesource.com/c/go/+/172078
Run-TryBot: Josh Bleecher Snyder <cdc248cc4d32e4fbfe04e45228163bcecd48a0b9@gmail.com>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

nawawi/go,CAFxX/go,CAFxX/go,cloudflare/go,josharian/go,josharian/go,josharian/go,CAFxX/go,akutz/go,golang/go,CAFxX/go,josharian/go,josharian/go,golang/go,nawawi/go,akutz/go,nawawi/go,golang/go,josharian/go,cloudflare/go,nawawi/go,cloudflare/go,golang/go,akutz/go,akutz/go,nawawi/go,cloudflare/go,golang/go,golang/go,CAFxX/go,nawawi/go,josharian/go,akutz/go,CAFxX/go,cloudflare/go,nawawi/go,akutz/go,golang/go,cloudflare/go,cloudflare/go,akutz/go,CAFxX/go,cloudflare/go,golang/go,nawawi/go,CAFxX/go,josharian/go,akutz/go