ar: handle partial writes from archive_write_data

[emaste]

Summary

It appears libarchive limits writes from a single archive_write_data() call to 0x7fffffff bytes.

from contrib/libarchive/libarchive/archive_write.c

        const size_t max_write = INT_MAX;
...
        /* In particular, this catches attempts to pass negative values. */
        if (s > max_write)
                s = max_write;
...
        return ((a->format_write_data)(a, buff, s));