Replace select() with poll() in non-threaded RTS

The non-threaded RTS still uses select() as of today, which obviously suffers from the FD_SETSIZE file descriptor count limit. We really should be using poll() which exists on all modern posix platforms (it's even supported by wasm32-wasi).

Tickets to look at before implementing: #635 (closed) #4934 (closed) #13497