The %S and %N format verbs are used by cmd/gc to
represent Sym and Node structures, respectively.

In liblink, these two verbs are used only by the %D
format routine and never referenced externally.

This change will allow us to delete the duplicated
code for the %A, %D, %P, and %R format routines in
both the compiler and linker.

R=golang-codereviews, rsc
CC=golang-codereviews
https://golang.org/cl/49720043