Fix: powerpc builds with linux v6.13

Fix: powerpc builds with linux v6.13

The crtsavres.o object for powerpc is missing the $(objtree) prefix.
This can be worked-around by setting MO=$KERNELDIR and building in the
kernel tree as before this version.

See upstream commit:

    commit 214c0eea43b2ea66bcd6467ea57e47ce8874191b
    Author: Masahiro Yamada <masahiroy@kernel.org>
    Date:   Sun Nov 10 10:34:30 2024 +0900

        kbuild: add $(objtree)/ prefix to some in-kernel build artifacts

        $(objtree) refers to the top of the output directory of kernel builds.

        This commit adds the explicit $(objtree)/ prefix to build artifacts
        needed for building external modules.

        This change has no immediate impact, as the top-level Makefile
        currently defines:

          objtree         := .

        This commit prepares for supporting the building of external modules
        in a different directory.

        Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
        Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>
Change-Id: I11f7ee1e7e5634e992953798e9ecef6b2558ff79
Signed-off-by: Kienan Stewart <kstewart@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>