fstream should not use BUFSIZ macro on windows. Instead it should set as 4096

[sotrdg sotrdg]

Stdio on windows uses 4096 as the real I/O buffer size.  However, BUFSIZ macro on MSVCRT and UCRT is wrongly set as 512 which causes massively slowing down of I/O performance on windows. I think std::filebuf and __gnu_cxx::stdio_filebuf should use 4096 as buffer size on windows.

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