I looked at the code change a bit more closely, and realized it's only implemented for int64 right now (I think). So I changed my Opcode type to int64 and re-ran my quick-n-dirty benchmark above, and now I'm seeing the jump tables in action. I've verified it's outputting an indirect jump (objdump shows "jmpq *(%rsi,%r12,8)").
Here are the results, showing the best of 5 runs. It's 10% faster for this code. Maybe not as big an improvement as I was hoping, but still pretty good!
$ time perflock ./goawk 'BEGIN { for (; i<10000000; i++) s+=i+i+i+i+i }'
real 0m0.853s
$ time perflock ./goawk_jumptables 'BEGIN { for (; i<10000000; i++) s+=i+i+i+i+i }'
real 0m0.765s
-Ben