Add ElfW() macro for compatibility with Linux

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, with a comment
that we prefer the ElfN() macro for FreeBSD.

Reviewed by: emaste, kib
Differential Revision: https://reviews.freebsd.org/D31427
MFC after: 1 week