gc: fix arm build

Escape analysis was incorrectly assuming that
functions without bodies don't leak their
parameters.  This meant that sync/atomic's
TestAddInt64 was allocating x on its stack,
and then x was not properly aligned for use
with the atomic 64-bit instructions.  Obviously
we should figure out the alignment story on 5g
too, but this fix is correct and should restore the
build to 'ok'.

TBR=lvd
CC=golang-dev
https://golang.org/cl/4964047

src/cmd/gc/esc.c

@@ -505,7 +505,7 @@ esccall(Node *n)
 		}
 	}
 			
-	if(fn && fn->op == ONAME && fn->class == PFUNC && fn->ntype) {
+	if(fn && fn->op == ONAME && fn->class == PFUNC && fn->defn && fn->defn->nbody && fn->ntype) {
 		// Local function.  Incorporate into flow graph.
 
 		// Receiver.
@@ -696,6 +696,10 @@ esctag(Node *func)
 {
 	Node *savefn;
 	NodeList *ll;
+	
+	// External functions must be assumed unsafe.
+	if(func->nbody == nil)
+		return;
 
 	savefn = curfn;
 	curfn = func;

test/escape2.go

@@ -774,3 +774,9 @@ func foo118(unknown func(*int)) {  // ERROR "unknown does not escape"
 	x := 1 // ERROR "moved to heap: NAME-x"
 	unknown(&x) // ERROR "&x escapes to heap"
 }
+
+func external(*int)
+
+func foo119(x *int) {  // ERROR "leaking param: NAME-x"
+	external(x)
+}