[0.2] TEEOS: Fix octal notation for O_* constants

[nathaniel-bennett]

Backport of #3840 for libc 0.2

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[tgross35]

Could you do the cherry-pick with -x so the commits wind up linked? In lieu of us being able to merge between the branches.

There are some directions at https://github.com/rust-lang/libc/blob/main/CONTRIBUTING.md#v10-roadmap

[nathaniel-bennett]

Cherry-picked now 👍

[tgross35]

Thanks!