Call FlushViewOfFile before unmapping the output file, for extra
safety. The documentation says the function does not wait for
the data to be written to disk, so it should be cheap.
Fixes #38440.
Change-Id: I05352f15d9305e6e7086a002f61802f74036b710
Reviewed-on: https://go-review.googlesource.com/c/go/+/235639 Reviewed-by: Austin Clements <austin@google.com> Reviewed-by: Jason A. Donenfeld <Jason@zx2c4.com>