Draft: Porting MMTK: Elimitating usage of `bdescr` and generations in non-sm part of RTS

Thank you for your contribution to GHC!

Please take a few moments to verify that your commits fulfill the following:

• are either individually buildable or squashed -- buildable with hadrian and works for trivial test fibo.hs
• have commit messages which describe what they do (referring to Notes and tickets using #NNNN syntax when appropriate) -- there are some commit messages
• have added source comments describing your change. For larger changes you likely should add a Note and cross-reference it from the relevant places.
• add a testcase to the testsuite. -- no testcase
• Currently working on preparation to port MMTK to GHC:
  MMTK has its sets of storage managements and GC plans. To port MMTK, the RTS should not have direct usage/assumptions on the structure of storage managements and GC algorithm. (e.g. usage of bdescr and RtsFlags.GcFlags.generations in the non-sm part of RTS).

If you have any questions don't hesitate to open your merge request and inquire in a comment. If your patch isn't quite done yet please do add prefix your MR title with WIP:.