Revert of [fullcodegen] Remove the hacky %_FastOneByteArrayJoin intrinsic.

src/runtime/runtime-array.cc

Index: src/runtime/runtime-array.cc
diff --git a/src/runtime/runtime-array.cc b/src/runtime/runtime-array.cc
index 125b87b9e76886e96ea70ec2a056acf476001c6f..09d48e986691a031045b8ad022a80832046779b6 100644
--- a/src/runtime/runtime-array.cc
+++ b/src/runtime/runtime-array.cc
@@ -489,6 +489,15 @@
 }
 
 
+RUNTIME_FUNCTION(Runtime_FastOneByteArrayJoin) {
+  SealHandleScope shs(isolate);
+  DCHECK(args.length() == 2);
+  // Returning undefined means that this fast path fails and one has to resort
+  // to a slow path.
+  return isolate->heap()->undefined_value();
+}
+
+
 RUNTIME_FUNCTION(Runtime_ArraySpeciesConstructor) {
   HandleScope scope(isolate);
   DCHECK(args.length() == 1);