DEBUG: crash using an invalid opcode on aarch64 instead of an invalid access

DEBUG: crash using an invalid opcode on aarch64 instead of an invalid access

On aarch64 there's also a guaranted invalid instruction, called UDF, and
which even supports an optional 16-bit immediate operand:

   https://developer.arm.com/documentation/ddi0596/2021-12/Base-Instructions/UDF--Permanently-Undefined-?lang=en

It's conveniently encoded as 4 zeroes (when the operand is zero). It's
unclear when support for it was added into GAS, if at all; even a
not-so-old 2.27 doesn't know about it. Let's byte-encode it.

Tested on an A72 and works as expected.