cmd/link: shrink SymKind down to a uint8
cmd/link: shrink SymKind down to a uint8

Change-Id: I54c73afd4fb2ad4d5854d2568524f57542c64fe5
Reviewed-on: https://go-review.googlesource.com/42182
Run-TryBot: Michael Hudson-Doyle <5e87ffb38a52af6fb9a19b679e9122c7a4d601a5@canonical.com>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

Cofyc/go,CAFxX/go,codestation/go,CAFxX/go,Cofyc/go,josharian/go,akutz/go,Cofyc/go,CAFxX/go,josharian/go,cloudflare/go,nawawi/go,nawawi/go,josharian/go,nawawi/go,golang/go,codestation/go,nawawi/go,akutz/go,cloudflare/go,cloudflare/go,josharian/go,Cofyc/go,Cofyc/go,golang/go,cloudflare/go,codestation/go,akutz/go,cloudflare/go,Cofyc/go,nawawi/go,josharian/go,CAFxX/go,nawawi/go,cloudflare/go,CAFxX/go,akutz/go,golang/go,cloudflare/go,akutz/go,Cofyc/go,codestation/go,akutz/go,akutz/go,josharian/go,CAFxX/go,cloudflare/go,christopher-henderson/Go,golang/go,golang/go,akutz/go,golang/go,codestation/go,codestation/go,josharian/go,CAFxX/go,golang/go,Cofyc/go,nawawi/go,codestation/go,golang/go,CAFxX/go,codestation/go,nawawi/go,josharian/go