I guess image.inc is for yocto image only, not for the toolchain
i don’t have any experience on building the toolchain, but here says you need to modify
meta-swi/common/recipes-core/meta/meta-toolchain-swi.bb
It also says " compiled binaries are located in ~/…/build_bin/tmp/sysroots/x86_64-linux/usr/bin/arm-poky-linux-gnueabi/arm-poky-linux-gnueabi-", I wonder if you can manually add your own library there and pack as a .sh file