syscall.Stat_t: consider shimming os.Ctime

Such that the following code (from go-git) compiles properly without requiring a separate logic for GopherJS:

```go
		if os, ok := sys.(*syscall.Stat_t); ok {
			// not supported on gopherjs:
			e.CreatedAt = time.Unix(int64(os.Ctime), int64(os.CtimeNsec))
                 }

```

Does this make sense to do?