cmd/gc: implement new return requirements

Fixes #65.

R=ken2
CC=golang-dev
https://golang.org/cl/7441049
diff --git a/test/fixedbugs/issue4663.go b/test/fixedbugs/issue4663.go
index b3d6602..edaee93 100644
--- a/test/fixedbugs/issue4663.go
+++ b/test/fixedbugs/issue4663.go
@@ -11,4 +11,5 @@
 
 func a(b int) int64 {
   b // ERROR "not used"
+  return 0
 }