powerpc and microblaze headers may require special additional config

[rofl0r]

musl-cross configures the kernel with a special default config on those archs.
we need to evaluate whether this affects the generated headers.

# Get our Linux arch and defconfig
LINUX_ARCH=`echo "$ARCH" | sed 's/-.*//'`
LINUX_DEFCONFIG=defconfig
case "$LINUX_ARCH" in
    i*86) LINUX_ARCH=i386 ;;
    arm*) LINUX_ARCH=arm ;;
    aarch64*) LINUX_ARCH=arm64 ;;
    mips*) LINUX_ARCH=mips ;;
    x32) LINUX_ARCH=x86_64 ;;

    powerpc* | ppc*)
        LINUX_ARCH=powerpc
        LINUX_DEFCONFIG=g5_defconfig
        ;;

    microblaze)
        LINUX_DEFCONFIG=mmu_defconfig
        ;;
esac
export LINUX_ARCH

...

    make $LINUX_DEFCONFIG ARCH=$LINUX_ARCH
    make headers_install ARCH=$LINUX_ARCH INSTALL_HDR_PATH="$CC_PREFIX/$TRIPLE"