Revamp autogeneration.  Get rid of gogenerate commands, they are more
trouble than they are worth.  (If the code won't compile, gogenerate
doesn't work.)

Generate opcode enums & tables.  This means we only have to specify
opcodes in one place instead of two.

Add arch prefixes to opcodes so they will be globally unique.

Change-Id: I175d0a89b701b2377bbe699f3756731b7c9f5a9f
Reviewed-on: https://go-review.googlesource.com/10812Reviewed-by: Alan Donovan <adonovan@google.com>