big endian ppc builds are broken

See for example: https://builds.tuxbuild.com/1liBNF6Jmycwvmc6DXUPeX4OB4p/build.log

It looks like powerpc64le-linux-gnu- is always used for the target triple, which will error when building for big endian targets (such as ppc44x_defconfig or pseries_defconfig). I suspect mips has a similar issue, where the arch is bi-endian and the target triple is must match the config).

For example, we might do the following builds:

$ ARCH=powerpc CROSS_COMPILE=powerpc64le-linux-gnu- make CC=clang -j powernv_defconfig
$ ARCH=powerpc CROSS_COMPILE=powerpc64-linux-gnu-   make CC=clang -j pseries_defconfig
$ ARCH=powerpc CROSS_COMPILE=powerpc-linux-gnu-     make CC=clang -j ppc44x_defconfig