syscall/js: show goroutine stack traces on deadlock
When using callbacks, it is not necessarily a deadlock if there is no
runnable goroutine, since a callback might still be pending. If there
is no callback pending, Node.js simply exits with exit code zero,
which is not desired if the Go program is still considered running.
This is why an explicit check on exit is used to trigger the "deadlock"
error. This CL makes it so this is Go's normal "deadlock" error, which
includes the stack traces of all goroutines.