cmd/link/internal/sym: add sizeof tests
cmd/link/internal/sym: add sizeof tests

CL 121916 showed that sym.Symbol matters for linker performance.
Prevent accidental regression.

Change-Id: I5fd998c91fdeef9e721bc3f6e30f775b81103e95
Reviewed-on: https://go-review.googlesource.com/122716
Run-TryBot: Josh Bleecher Snyder <cdc248cc4d32e4fbfe04e45228163bcecd48a0b9@gmail.com>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

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