Use Cmake to generate files from codec.txt automatically on AArch64.

Created by: fhahn

This patch removes

• core/arch/aarch64/decode_gen.h,
• core/arch/aarch64/encode_gen.h
• core/arch/aarch64/opcode.h
• core/arch/aarch64/opcode_names.h

and instead adds Cmake rules to automatically generate them from codec.txt.

The main advantage is that this dramatically reduces the diff for codec changes on AArch64, as it will not contain tons of changes in the auto-generated files.

Change-Id: I88c38835c887d252fe0fa4e406879e3b5d682c19