Use `getentropy()` instead of `/dev/urandom` on Emscripten
`/dev/urandom` is usually available on Emscripten, except when using
the special `NODERAWFS` filesystem backend, which replaces all normal
filesystem access with direct Node.js operations.
Since this filesystem backend directly access the filesystem on the
OS, it is not recommended to depend on `/dev/urandom`, especially
when trying to run the Wasm binary on OSes that are not Unix-based.
This can be considered a non-functional change, since Emscripten
implements `/dev/urandom` in the same way as `getentropy()` when not
linking with `-sNODERAWFS`.
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