package/pkg-cmake: set CMAKE_SYSTEM_PROCESSOR correctly on powerpc64{, le}

package/pkg-cmake: set CMAKE_SYSTEM_PROCESSOR correctly on powerpc64{, le}

Like ARM, BR2_ARCH doesn't correspond to the uname -m. With cmake
CMAKE_SYSTEM_PROCESSOR is expected to contain the uname -m value.

So we change CMAKE_SYSTEM_PROCESSOR from powerpc64 -> ppc64 (big endian)
and powerpc64le -> ppc64le (little endian).

This corrects the cross compile to the powerpc64{,le} target
architecture for the mariadb (and possibly others) packages that
use the CMAKE_SYSTEM_PROCESSOR variable and compare it against
Power64 based architectures.

Signed-off-by: Daniel Black <daniel.black@au.ibm.com>
Acked-by: Cyril Bur <cyrilbur@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>