30.5.2. ARM UDF instruction
Guaranteed undefined! Therefore raise illegal instruction signal. Used by GCC __builtin_trap apparently: https://stackoverflow.com/questions/16081618/programmatically-cause-undefined-instruction-exception
Why GNU GAS 2.29 does not have a mnemonic for it in A64 because it is very recent: shows in ARMv8 architecture reference manual db but not ca.