syscall: remove support for O_NONBLOCK and O_SYNC on js/wasm
This commit removes O_NONBLOCK on js/wasm. O_SYNC can't be
removed, because it is referenced by the os package, so instead
its use returns an error.
On Windows, the options O_NONBLOCK and O_SYNC are not available
when opening a file with Node.js. This caused the initialization
of the syscall package to panic.
The simplest solution is to not support these two options on js/wasm
at all. Code written for js/wasm is supposed to be portable,
so platform-specific options should not be used.