Modify 'autotools/convert-constants' not to generate the Haskell
constant that holds all opcode names.  By having Haskell generate the
Python opcopdes, it becomes impossible to generate this constant due
to a cyclic dependency.  However, this constant is used only in tests,
therefore, it can be generated from Template Haskell. This patch also
updates the test to execute a Python program to fetch the generated
opcode names instead of relying on the beforementioned constant.

Signed-off-by: Jose A. Lopes <jabolopes@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>