package/gcc: restrict gcc 8.x to PowerPC SPE

package/gcc: restrict gcc 8.x to PowerPC SPE

gcc 11.1 is around, gcc 10.2 is the default version, so drop
8.4 in order to reduce the gcc choice.

For PowerPC SPE, introduce BR2_GCC_VERSION_POWERPC_SPE
to avoid removing defconfigs arcturus_ucp1020_defconfig,
freescale_p1025twr_defconfig and qemu_ppc_mpc8544ds_defconfig
that are still used by some users [1].
BR2_GCC_VERSION_POWERPC_SPE use the same gcc version (8.4), no
change expected.

[1] 96e80ad214ced86c802d561946fa7da88c1595eb

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: Oleksandr Zhadan <oleks@arcturusnetworks.com>
Cc: Michael Durrant <mdurrant@ArcturusNetworks.com>
Cc: Matthew Weber <matthew.weber@collins.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>