fix std build on nuttx

[RalfJung]

#163024 accidentally broke that build. Apparently there is no AT_SYMLINK_NOFOLLOW.
Cc @no1wudi

It seems like nuttx actually does have the flag though, it's just missing from libc? I filed an issue for that: rust-lang/libc#5599.

[rustbot]

r? @clarfonthey

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[clarfonthey]

Since this is a tier 3 target, I'm tempted to just close this until the libc issue is resolved.

[RalfJung]

Yeah that would make the existing code just work. When I started with this PR I did not realize that it was just missing from libc.

[clarfonthey]

I'll close this, then.