cmd/gc: don't allow escaping arguments in the runtime package.

This is a case that was missed in https://golang.org/cl/105280047/

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/120910043

src/cmd/gc/walk.c

@@ -2356,6 +2356,8 @@ paramstoheap(Type **argin, int out)
 			continue;
 
 		// generate allocation & copying code
+		if(compiling_runtime)
+			fatal("%N escapes to heap, not allowed in runtime.", v);
 		if(v->alloc == nil)
 			v->alloc = callnew(v->type);
 		nn = list(nn, nod(OAS, v->heapaddr, v->alloc));