cmake: Use consistent arch names for POWER targets

cmake: Use consistent arch names for POWER targets

The arch detection code used by cmake build exports ppc arch names in an
abbreviated way, e.g. ppc64le instead of powerpc64le. Some parts of
CMakeLists.txt are using the longer form instead, which will not
properly match the desired subarch as expected. Switch to using the
longer form everywhere for consistency with 'configure' script and with
arch names obtained when running under QEMU.