runtime: don't allocate a new string in printf

LGTM=dave
R=rsc, dave
CC=golang-codereviews
https://golang.org/cl/139890043
diff --git a/src/pkg/runtime/stubs.go b/src/pkg/runtime/stubs.go
index 26126fc..2014dfb 100644
--- a/src/pkg/runtime/stubs.go
+++ b/src/pkg/runtime/stubs.go
@@ -203,6 +203,7 @@
 func lock(lk *mutex)
 func unlock(lk *mutex)
 func purgecachedstats(c *mcache)
+func gostringnocopy(b *byte) string
 
 //go:noescape
 func write(fd uintptr, p unsafe.Pointer, n int32) int32