Log Message:
Add support for building ocaml on NetBSD/powerpc.

On NetBSD 8.x, there will be issues with shared library
support, due to missing support for ADDR16_LO, ADDR16_HI
and ADDR16_HA relocations, available in newer code.
There are however other issues, text relocations, and
apparent missing REL24 relocation support(?!?).

The testyield.ml test appears to not terminate, only two
of the 4(?) threads ever get any CPU time, so has to be killed
to complete the selftests.  Test result:

  2438 tests passed
   36 tests skipped
   12 tests failed
  108 tests not started (parent test skipped or failed)
    8 unexpected errors
  2602 tests considered