overhauls the target/architecture abstraction (3/n)

[ivg]

In this episode, we liberate bap mc and bap objdump from the bonds
of the Arch.t representation. We also add the systemz lifter for
demonstration purposes. Of course, the lifter is minimal and far from
being usable, but that serves well its didactic purposes.

The interface of the bap mc command is preserved but is extended
with a few more command-line options that provide a great deal of
flexibility. Not only it is now possible to specify the target and
encoding, but it is now possible to pass options directly to the
backend, which is useful for disassembling targets that are not yet
known to BAP. Below is an excerpt from the bap-mc man page
(see bap mc --help)

       SETTING ARCHITECHTURE

       The target architecture is controlled by several groups of options that
       can not be used together:

       - arch;
       - target and encoding;
       - triple, backend, cpu, bits, and order.

       The arch option provides the least control but is easiest to use. It
       relies on the dependency-injection mechanism and lets the target
       support packages (plugins that implement support for the given
       architecture) do their best to guess the target and encoding that
       matches the provided name. Use the common names for the architecture
       and it should work. You can use the bits and order options to give more
       hints to the target support packages. They default to 32 and little
       correspondingly.

       The target and encoding provides precise control over the selection of
       the target and the encoding that is used to represent machine
       instructions. The encoding field can be omitted and will be deduced
       from the target. Use  bap list targets and  bap list encodings to get
       the list of supported targets and encodings respectivly.

       Finally, the triple, backend, cpu,... group of options provides the
       full control over the disassembler backend and bypasses the
       dependency-injection mechanism to pass the specified options directly
       to the corresponding backends. This enables disassembling of targets
       and encodings that are not yet supported by BAP. The meanings of the
       options totally depend on the selected backend and they are passed as
       is to the corresponding arguments of the Disasm_expert.Basic.create
       function. The bits and order defaults to 32 and little corresondingly
       and are used to specify the number of bits in the target's addresses
       and the order of bytes in the word. This group of options is useful
       during the implementation and debugging of new targets and thus is
       reserved for experts. Note, when this group is used the semantics of
       the instructions will not be provided as it commonly requires the
       target specification.

[ivg]

As a small teaser, I will finish it on Monday (need a bit more documentation and polishing, probably will add a couple more instruction to the systemz lifter).