Add third-party verification tests for AArch64 decoder/encoder

The current test set for the AArch64 decoder/encoder checks internal consistency but not correctness against an external third-party reference decoder/encoder. For AArch64, GNU's binutils and/or QEMU could be used for such cross-checking.

The x86 decoder/encoder already does this using binutils and udis86.

This issue was raised as a result of discussions in #4395 , see https://github.com/DynamoRIO/dynamorio/pull/4395#discussion_r466503441.