Add ElfW() macro for compatibility with Linux.

[dim]

Summary

Some Linux software using ELF headers assumes the existence of an
ElfW(type) macro, which concatenates 'Elf', the default ELF word size,
and the given type. This is identical to our __ElfN(x) macro in
<sys/elf_generic.h>. Add the macro for compatibility.

Test Plan

maybe this needs an exp-run?

[emaste]

I think an exp-run is worthwhile

[kib]

sys/sys/elf_generic.h

60

I think some recommendation would be useful, like

.. with Linux, prefer __ElfN() in FreeBSD code */