Experimental support for RTS-only "ways"

HEADS UP!  This changes the way that the threaded RTS is used, and
also the use of debugging RTSs:

  - We always build threaded and debugging variants of the RTS now.
    The --enable-threaded-rts configure option is ignored (and will
    be removed at some point).

  - New option:  -debug     enables the debugging RTS

  - New option:  -threaded  enables the threaded RTS.  When the threaded
    RTS is stable enough, we might make it the default.

The new options just cause a different variant of the RTS to be linked
in, and they cause one or two extra options to be enabled too.  The
implementation is via the usual ways machinery in the compiler, except
that these ways are labelled as RTS-only, and so don't require
rebuilding all the libraries too.

All of this means we can ship threaded and debugging RTSs with GHC, so
that users don't need to fetch and build a GHC source tree to use
them.

I'd like to get this functionality into 6.2.1 if possible, so please
test (I'm willing to stretch the definition of "interface change" to
accomodate this, since having a threaded RTS available without having
to build GHC will be a big win for the Visual Studio project).