Hello, I believe this is related to insufficient CHOST matching in bootstrap-rap.sh (which seems to be done similarly to how crossdev does it). powerpc64le-* does not match powerpc64-* (likewise, armv7em does not match armv7). I solved the issue by linking $EPREFIX/usr/portage/profiles/prefix/linux-standalone/ppc64/ to make.profile. R0b0t1.
profiles/default/linux/ppc64le/17.0/prefix exists nowadays