cmd/gc: make runtime escape an error, not a fatal error

It is more useful to report all the errors instead of just the first.

LGTM=dave, khr
R=khr, dave
CC=golang-codereviews
https://golang.org/cl/143940043

src/cmd/gc/gen.c

@@ -585,7 +585,7 @@ cgen_dcl(Node *n)
 	if(!(n->class & PHEAP))
 		return;
 	if(compiling_runtime)
-		fatal("%N escapes to heap, not allowed in runtime.", n);
+		yyerror("%N escapes to heap, not allowed in runtime.", n);
 	if(n->alloc == nil)
 		n->alloc = callnew(n->type);
 	cgen_as(n->heapaddr, n->alloc);

src/cmd/gc/walk.c

@@ -2505,7 +2505,7 @@ paramstoheap(Type **argin, int out)
 
 		// generate allocation & copying code
 		if(compiling_runtime)
-			fatal("%N escapes to heap, not allowed in runtime.", v);
+			yyerror("%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));