liblink: allow either of a pair of conflicting symbols to say dupok
liblink: allow either of a pair of conflicting symbols to say dupok

This makes the linker's -X flag work again.

R=iant
CC=bradfitz, golang-codereviews, golang-dev
https://codereview.appspot.com/44360043

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang