package/gcc: don't build a toolchain for powerpc with spe ABI with gcc >= 8.x

package/gcc: don't build a toolchain for powerpc with spe ABI with gcc >= 8.x

The powerpc*-*-*spe* support has been deprecated in GCC 8 [1] and has
been removed in GCC 9 [2].

While building with GCC 8, the build stop since we don't provide --enable-obsolete
option.

For now, keep powerpcspe support in Buildroot but only for older compiler
up to GCC 7.

Note: Although we can't select Glibc for powerpcspe since commit [4], this
support has been removed for the next version of Glibc (the upcoming 2.30).

[1] https://gcc.gnu.org/ml/gcc/2018-04/msg00102.html
[2] https://gcc.gnu.org/git/?p=gcc.git;a=commitdiff;h=b31d0348ddada49453e3edaaf93a423fdc61dc79
[3] https://sourceware.org/git/?p=glibc.git;a=commit;h=a053e878494080f7070cf92890e546057236c9c9
[4] https://git.buildroot.net/buildroot/commit/?id=5777e3ffd99132a57e3d29659850eec14b1625f8

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Reviewed-by: Matt Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>