LibC: Flush all file streams on exit

The POSIX standard specifies the following:

> If the main() function returns to its original caller, or if the
> exit() function is called, all open files are closed (hence all output
> streams are flushed) before program termination.

This means that flushing `stdin` and `stdout` only is not enough, as the
program might have pending writes in other file buffers too.

Now that we support `fflush(nullptr)`, we call that in `exit()` to flush
all streams. This fixes one of bash's generated headers not being
written to disk.

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