compat_binfmt_elf: don't bother with undef of ELF_ARCH

It's not used anywhere downstream (and never had been, AFAICS).
Theoretically, fs/binfmt_elf.c does use it, but only in the
non-regset coredump handling and all biarch architectures
end up with that ifdefed out.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>