[OpenMP][JIT] Introduce more debugging configuration options

The JIT is a great debugging tool since we can modify the IR manually
before launching it in an existing test case. The new flasks allow to
skip optimizations, to use the exact given IR, as well as to provide a
finished object file. The latter is useful to try out different backend
options and to have complete freedom with pass pipelines.

Documentation is included. Minimal refactoring was performed to make the
second object fit in nicely.