crypto/o_fopen.c: alias fopen to fopen64.

Originally fopen(3) was called from bio/bss_file.c, which performed the
aliasing. Then fopen(3) was moved to o_fopen.c, while "magic" definition
was left behind. It's still useful on 32-bit platforms, so pull it to
o_fopen.c.

Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: Tim Hudson <tjh@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6596)