ELF64. On other architectures only include a.out support, since we don't
know whether or not they'll have the headers necessary for ECOFF and/or
ELF, and since they only want a.out for now (execpt the mips archs, but
they do Special things). There should be a better way to select which
bits of nlist code get compiled in, but currently there is not.
(1) split nlist() into multiple files, for clarity and to make
ELF 32/64 support easier,
(2) support multiple executable types at the same time, and
(3) add support for 32- and 64-bit ELF (32-bit ELF support
originally from OpenBSD, but with several bug fixes so
that it actually handles symbols types more correctly
(and therefore _works_ for some of the more tricky uses
of the nlist routines) and changes for 64-bit ELF support).
bjh21