JS: implementation of openat is missing

The JS backend doesn't support the openat libc function.

NodeJS doesn't provide it so I'm not sure we can fully support it (see https://github.com/nodejs/node/issues/31110). However we can support the case where dirfd is set to AT_FDCWD by falling back to using open.

This was found while trying to make a testcase for https://github.com/haskell/unix/pull/285: createFile calls openat (with dirfd set to AT_FDCWD).

Edited Jul 19, 2023 by Sylvain Henry

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