cmd/compile: deduplicate symbol references

cmd/compile: deduplicate symbol references

Reduces size of archives in pkg/linux_amd64 by 1.4MB (3.2%),
slightly improving link time.

name       old s/op   new s/op   delta
LinkCmdGo  0.52 ± 3%  0.51 ± 2%  -0.65%  (p=0.000 n=98+99)

Change-Id: I7e265f4d4dd08967c5c5d55c1045e533466bbbec
Reviewed-on: https://go-review.googlesource.com/20802
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>