qemu.bbclass: Extend ppc/ppc64 extra options

qemu.bbclass: Extend ppc/ppc64 extra options

Some recipes are marked machine specific which need qemu usermode during
build eg. if they use meson build system, which means they wont get
right -cpu settings to run qemu-ppc/qemu-ppc64 and build fails, this
ensures that we set the right options when PACKAGE_ARCH is set to
MACHINE_ARCH on ppc/ppc64 qemu

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>