Switch Fuchsia to readdir (instead of readdir_r)
The readdir_r function is deprecated on newer Posix systems because of
various problems, and not implemented at all for Fuchsia. There are
already implementations using both, and this patch switches Fuchsia
over to the readdir-based one.
Fixes#40021 for Fuchsia, but that issue also contains discussion of
what should happen for other Posix systems.
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