Make build-many-glibcs.py build GCC for powerpcspe with --enable-obsolete.

The powerpcspe GCC port has been obsoleted in GCC 8 for not having had
the removal of code for non-SPE processors completed.  This patch
accordingly arranges for build-many-glibcs.py to configure GCC with
--enable-obsolete for affected configurations.  This is temporary;
either the port gets cleaned up and unobsoleted in GCC and the
configure option can be removed, or the port gets removed in GCC and
we should remove the corresponding glibc support.

Tested with build-many-glibcs.py for the affected configurations.

	* scripts/build-many-glibcs.py (Context.add_all_configs): Use
	--enable-obsolete for powerpc-linux-gnuspe.