[dev.cc] runtime: convert Plan 9 port to Go

Thanks to Aram Hăvărneanu, Nick Owens
and Russ Cox for the early reviews.

LGTM=aram, rsc
R=rsc, lucio.dere, aram, ality
CC=golang-codereviews, mischief
https://golang.org/cl/175370043
diff --git a/src/runtime/env_plan9.go b/src/runtime/env_plan9.go
index e442c34..ec50cac 100644
--- a/src/runtime/env_plan9.go
+++ b/src/runtime/env_plan9.go
@@ -54,3 +54,6 @@
 	sp.len = int(r)
 	return s
 }
+
+var _cgo_setenv unsafe.Pointer   // pointer to C function
+var _cgo_unsetenv unsafe.Pointer // pointer to C function