cmd/link: make sure we're hashing __.PKGDEF in genhash
cmd/link: make sure we're hashing __.PKGDEF in genhash

This is currently always the case because loadobjfile complains if
it's not, but that will be changed soon.

Updates #24512.

Change-Id: I262daca765932a0f4cea3fcc1cc80ca90de07a59
Reviewed-on: https://go-review.googlesource.com/102280
Run-TryBot: Matthew Dempsky <e56ac2c500064424afac424d4b5e53bd0570a731@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

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