cmd/gc: mark LOCALS argument as a constant to print correctly

Fixes #4875.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/7376049

src/cmd/gc/pgen.c

@@ -135,6 +135,7 @@ compile(Node *fn)
 	oldstksize = stksize;
 	allocauto(ptxt);
 
+	plocals->to.type = D_CONST;
 	plocals->to.offset = stksize;
 
 	if(0)