sdk: include lib/crtsavres.o for powerpc

sdk: include lib/crtsavres.o for powerpc

Trying to link certain kernel modules like dahdi-linux when building with
the OpenWrt SDK will fail with:
openwrt-sdk-apm821xx-sata_gcc-13.2.0_musl.Linux-x86_64/staging_dir/toolchain-powerpc_464fp_gcc-13.2.0_musl/bin/powerpc-openwrt-linux-musl-ld: cannot find arch/powerpc/lib/crtsavres.o: No such file or directory

Previously this worked with the PowerPC SDK since we carried a hack that
was passing --save-restore-funcs to module LDFLAGS so the linker provided
the required functions automatically as without --save-restore-funcs it
doesnt do so automatically on relocatable links and as a sideffect did not
require the kernel provided crtsaves.o to link against.

Now that hack has been removed as upstream kernel now compiles crtsaves.o
by default so it can be linked against but its not included in the SDK.

So, lets include lib/crtsavres.o when SDK is generated for PowerPC.

Fixes: 99c9d8abd677 ("kernel: bump 5.15 to 5.15.148")
Signed-off-by: Robert Marko <robimarko@gmail.com>