classes/qemu: move QEMU_EXTRAOPTIONS for PPC to the relevant tunes

classes/qemu: move QEMU_EXTRAOPTIONS for PPC to the relevant tunes

Every other architecture has the QEMU_EXTRAOPTIONS assignments in the
tune files, so move the PPC ones too.

Leave the MACHINE_ARCH workarounds present for now as these are masking
a genuine bug (#15647) that needs to be resolved properly.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>