[patch] Let stddef.h redefine NULL if __need_NULL is set

[Nico Weber]

>> Let me know which one of the 3 patches (with 1 being the one I
>> originally sent) you like best. If it's 3, I'll add a few more tests
>> for that.
>
>
> I strongly prefer patch 3 over 2. (With just patch 2, I worry that some libc
> header would say "give me NULL and FOO", and just get NULL.)
>
> Instead of repeating an 'ifdef _STDDEF_H_imp' test, could you define all the
> __need_* macros in the _STDDEF_H_imp section? I'd find that slightly easier
> to reason about.

Done. Also added a test, and fixed bugs found by the test.
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