Same procedure as every year: GCC defaults change (GCC 9)

GCC 9 was released earlier this year, it is now available in Debian
testing/unstable. I am planning to do the defaults change in mid August, around
the time of the expected first GCC 9 point release (9.2.0).

There are only soname changes for rather unused shared libraries (libgo)
involved, and the gnat defaults change will be handled separately by the Debian
Ada maintainers.  The fortran module changes look ok according to Alastair
McKinstry.

The gcc-9 package still ftbfs on kfreebsd-*.

We still have local patches for at least the various mips, kfreebsd and hurd
targets.  Please forward these upstream and make sure that these are applied
upstream.

powerpcspe support is removed upstream.  I will keep pointing the default to GCC
8 for this target.

Matthias