aarch64: Use an enum to refer to indices in the opcode table

aarch64: Use an enum to refer to indices in the opcode table

The indices into the auto-generated tables for opcodes are relatively
unstable.  Adding a new opcode can permute the code significantly.
But most of this churn is down to changes in the index values.  To
minimize this use enumerated constants.  While the index values
change, the enumeration names will need to do so far less often, so
most of the changes in the generated code become localized to the
addition (occasionally removal) of opcodes.  This change also makes
the state-change comments unnecessary.  The enumeration names contain
the same information (and more), so these are simply deleted.

The enumeration values are placed in a new header file, aarch64-tbl-2.h,
so aarch64-gen gains a new option to build this header and the Makefile
rules are adjusted accordingly.