libroot: os/arch: remove unsupported architectures
Remove unused Haiku64Image
Remove no longer needed HAIKU_STRIP_DEBUG_FROM_OPTIONAL_PACKAGES
More work towards hybrid support

* All packaging architecture dependent variables do now have a
  respective suffix and are set up for each configured packaging
  architecture, save for the kernel and boot loader variables, which
  are still only set up for the primary architecture.
  For convenience TARGET_PACKAGING_ARCH, TARGET_ARCH, TARGET_LIBSUPC++,
  and TARGET_LIBSTDC++ are set to the respective values for the primary
  packaging architecture by default.
* Introduce a set of MultiArch* rules to help with building targets for
  multiple packaging architectures. Generally the respective targets are
  (additionally) gristed with the packaging architecture. For libraries
  the additional grist is usually omitted for the primary architecture
  (e.g. libroot.so and <x86>libroot.so for x86_gcc2/x86 hybrid), so that
  Jamfiles for targets built only for the primary architecture don't
  need to be changed.
* Add multi-arch build support for all targets needed for the stage 1
  cross devel package as well as for libbe (untested).