Support NetBSD m68K ELF targets

* mpi/config.links (m68k*-*-netbsdelf): New.

The original patch from NetBSD had this additional code:

if ! test x$M68060 = xYES; then

	    path="m68k/mc68020 m68k"

fi

However, this bypasses the config system. It would be better to add
proper support for this CPU to config.sub.