bitbake.conf: override -cpu for qemu on ppc with SPE

bitbake.conf: override -cpu for qemu on ppc with SPE

We use -mtune flags which lets our application have
SPE intruction in them which is a good thing but when
running qemu especially in user mode these application
wont work with default powerpc emulation. Therefore we
have to add proper -cpu option to let qemu emulate for
a given cpu.

(From OE-Core rev: 695598415b4e9e17def7f04332bdbc0d24cc1efc)

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>