conf/machine-sdk: Add ppc64le SDK machine

Previously SDKMACHINE would default to x86_64. Commit 539cd4a5
("bitbake.conf: default SDKMACHINE to the build host architecture")
changed it to default to the build machine, causing the build to
fail on a ppc64le host.

Reported-by: Andrew Geissler <geissonator@yahoo.com>
Link: https://www.mail-archive.com/yocto@lists.yoctoproject.org/msg04364.html


Signed-off-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>