mongodb: failes to build on ppc

Checking if C compiler "powerpc-poky-linux-gcc  -m32 -mhard-float -mcpu=7400 -mno-spe --sysroot=/home/..../poky/build/tmp/sysroots/qemuppc" is GCC... yes
 | KeyError: 'powerpc':

add powerpc to COMPATIBLE_MACHINE.

Signed-off-by: Armin Kuster <akuster808@gmail.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>