Assume unknown operating systems use ELF.
There's a lot of new Unix-like operating systems out that that might
port LibreSSL (my Sortix is among them) and they all use ELF. If the
operating system doesn't use ELF or isn't an Unix, it doesn't have a
chance at working out of this box anyway, and this change makes
LibreSSL work on a generic and sufficiently featured POSIX system.
