base: treat all FDs as "nonblocking" on wasm

On posix platforms, when performing read/write on FDs, we check the nonblocking flag first. For FDs without this flag (e.g. stdout), we call fdReady() first, which in turn calls poll() to wait for I/O to be available on that FD. This is problematic for wasm32-wasi: although select()/poll() is supported via the poll_oneoff() wasi syscall, that syscall is rather heavyweight and runtime behavior differs in different wasi implementations. The issue is even worse when targeting browsers, given there's no satisfactory way to implement async I/O as a synchronous syscall, so existing JS polyfills for wasi often give up and simply return ENOSYS.

Before we have a proper I/O manager that avoids poll_oneoff() for async I/O on wasm, this patch improves the status quo a lot by merely pretending all FDs are "nonblocking". Read/write on FDs will directly invoke read()/write(), which are much more reliably handled in existing wasi implementations, especially those in browsers.

Fixes #23275 (closed) and the following test cases: T7773 isEOF001 openFile009 T4808 cgrun025