stdio: make stdout block-buffered when not associated to a terminal

[andreacorbellini]

This changes std::io::Stdout so that:

• it is line-buffered when connected to a terminal
• it is block-buffered otherwise

This matches the behavior of standard library implementations from other popular languages (C/C++/Python just to name a few).

[rustbot]

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[andreacorbellini]

This PR seems to have discovered an interesting bug. Reported at #148278

[the8472]

Please see #60673 for prior discussion