all: implement trivial select statements

Implement two trivial uses of the select statement.

Always blocking:
    select {}

No-op:
    select {
    default:
    }

Go 1.12 added a `select {}` instruction to syscall/js, so this is needed
for Go 1.12 support. More complete support for select will be added in
the future.

src/runtime/chan.go

@@ -41,6 +41,7 @@ const (
 
 func chanSendStub(caller *coroutine, ch *channel, _ unsafe.Pointer, size uintptr)
 func chanRecvStub(caller *coroutine, ch *channel, _ unsafe.Pointer, _ *bool, size uintptr)
+func deadlockStub()
 
 // chanSend sends a single value over the channel. If this operation can
 // complete immediately (there is a goroutine waiting for a value), it sends the