kbuild: Add missing $(objtree) prefix to powerpc crtsavres.o artifact

In the upstream commit 214c0eea
("kbuild: add $(objtree)/ prefix to some in-kernel build artifacts")
artifacts required for building out-of-tree kernel modules had
$(objtree) prepended to them to prepare for building in other
directories.

When building external modules for powerpc,
arch/powerpc/lib/crtsavres.o is required for certain
configurations. This artifact is missing the prepended $(objtree).

Fixes: 13b25489 ("kbuild: change working directory to external module directory with M=")
Acked-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <n.schier@avm.de>
Tested-by: Nicolas Schier <n.schier@avm.de>
Signed-off-by: Kienan Stewart <kstewart@efficios.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20250218-buildfix-extmod-powerpc-v2-1-1e78fcf12b56@efficios.com