eliminate the unnecessary and especially confusing concept of a 'code_generator' an isa-interface is enough

[r28009]

[r27454]

- Add a generic requirements+register+flag dumper to bearch; This avoids all
  backends/benode having their own slightly different dumpers
- Lots of cleanups in the backends along the way

[r26542]

[r17143]

[r13566]

[r13508]