Don't buffer logs on Win32
Don't buffer logs on Win32

Fixes #1042 on Windows.

Win32 doesn't support line buffering (`_IOLBF`) on streams:
https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/setvbuf?view=vs-2019#remarks

Ensuring useful log lines are written on a crash is much more
difficult without line buffering. Arbitrary buffer sizes will still
cut off log lines unless they're so small that the performance
difference compared to not buffering isn't much.

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