Instead of playing #include games to get a 32-bit ELF implementation on

Instead of playing #include games to get a 32-bit ELF implementation on
amd64, simply compile in our own nlist_elf32. Remove ECOFF and AOUT from
the nlist implementation since we do not need it.

This makes the code identical for the i386 and amd64 versions of
installboot(8).