[dev.cc] cmd/go: install cmd/asm as a tool

cmd/dist was doing the right thing, but not cmd/go.

Change-Id: I5412140cfc07e806152915cc49db7f63352d01ca
Reviewed-on: https://go-review.googlesource.com/5451
Reviewed-by: Rob Pike <r@golang.org>

src/cmd/go/pkg.go

@@ -393,6 +393,7 @@ const (
 var goTools = map[string]targetDir{
 	"cmd/addr2line":                        toTool,
 	"cmd/api":                              toTool,
+	"cmd/asm":                              toTool,
 	"cmd/cgo":                              toTool,
 	"cmd/dist":                             toTool,
 	"cmd/fix":                              toTool,