fix arm a_crash for big endian

fix arm a_crash for big endian

contrary to commit 89e149d275a7699a4a5e4c98bab267648f64cbba, big
endian arm does need the instruction bytes in big endian order. rather
than trying to use a special encoding that works as arm or thumb,
simply encode the simplest/canonical undefined instructions dependent
on whether __thumb__ is defined.