MmFile.flush is broken on windows

[Temtaime]

https://github.com/dlang/phobos/blob/master/std/mmfile.d#L424

flush does not call FlushFileBuffers therefore does not really flush.

https://learn.microsoft.com/en-us/windows/win32/api/memoryapi/nf-memoryapi-flushviewoffile:

To flush all the dirty pages plus the metadata for the file and ensure that they are physically written to disk, call FlushViewOfFile and then call the [FlushFileBuffers](https://learn.microsoft.com/en-us/windows/desktop/api/fileapi/nf-fileapi-flushfilebuffers) function.

[thewilsonator]

fixed by #10878