Have ___sys_statfs64 return real values when the FS supports it

Version of emscripten/emsdk: 3.1.61

The ___syscall_statfs64 function currently returns hardcoded values while some filesystems could potentially return more accurate stats. For example, NODEFS could leverage the Node statfs filesystem function to return actual stats for a given mount point.

Is this something worth considering or are there any problems with such an approach?

This sounds reasonable to me. Would you have time to send a PR perhaps?