Deprecation of powerpc*-*-*spe*

[Jakub Jelinek]

As has been discussed in the
https://gcc.gnu.org/ml/gcc/2017-02/msg00041.html
https://gcc.gnu.org/ml/gcc-patches/2017-05/msg01227.html
https://gcc.gnu.org/ml/gcc-patches/2018-04/msg00810.html
threads and in
https://gcc.gnu.org/PR81084
the powerpc*-*-*spe* support which has been split into a separate backend
hasn't been given enough maintainance and so is deprecated in GCC 8.

It can still be built with --enable-obsolete.  If the maintainers don't
manage to remove the dead parts of the port and start actually maintaining
it early in the GCC 9 stage1 development, the port will be removed.