1.0.28.77: fix test failures on PPC and elsewhere

1.0.28.77: fix test failures on PPC and elsewhere

* Most of the failures were test that cannot pass due to missing
  bits of DX implementation -- stack allocatable vectors and
  fixed-alloc.

* Mark backtrace test 353 as expected to fail on PPC as well.

* Don't declare *HANDLER-CLUSTERS* as dynamic-extent on platforms
  that do not support DX fixed-alloc, since it will just give
  a spurious compiler note.