AArch64 codec is not scalable

The current methods of defining and generating encode and decode functionality are not scalable for implementing support of ISA versions after v8.0

At present AArch64 has one codec.txt and one codec.c file which defines all instructions currently supported as well as all the bitmask and operand definitions. Encode and decode data and code generated from codec.txt are also all in one set of files regardless of ISA version.

Test files like dis-a64.txt and ir_aarch64.c contain all encode/decode instructions tests.

This issue will track separation of the codec by ISA version for the purposes of reduced maintenance overhead and more reliable functionality.