package/gcc: re-enable gcc 8.x for PowerPC SPE

package/gcc: re-enable gcc 8.x for PowerPC SPE

The GCC-7.x compiler series was the last to officially support PowerPC
SPE CPUs. Now that GCC-8.x is the default compiler used by Buildroot,
some defconfigs, notably the arcturus_ucp1020_defconfig and
freescale_p1025twr_defconfig ones started to fail building, as they
are PowerPC SPE platforms.

In fact, the GCC-8.x compiler series continues to support PowerPC SPE
CPU cores, but only as an --enable-obsoleted instruction set. This
patch enables the use of GCC-8.x and asserts the required option to
enable the PowerPC SPE instruction set.

This patch passes compilation and run tests with the
arcturus/ppc-ucp1020  board.

This patch should address a noted job failure on GitLab CI
https://gitlab.com/buildroot.org/buildroot/-/jobs/805461732

Fixes: https://gitlab.com/buildroot.org/buildroot/-/jobs/805461732
Signed-off-by: Michael Durrant <mdurrant@ArcturusNetworks.com>
Signed-off-by: Oleksandr G Zhadan <Oleks@ArcturusNetworks.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>