sys-apps/portage is missing a dependency on libgcc_s.so.1.
similarly to bug 740744, this means that bootstrapping new system e.g. with:
emerge --root /new/gentoo/system portage
fails, since running portage from the resulting tree causes portage exit with an error:
libgcc_s.so.1 must be installed for pthread_cancel to work
Emerging sys-devel/gcc into the new system fixes this problem.
Wouldn't this rather be a dependency of the python interpreter?
In case it helps for your use case, I would typically use these options for the type of command that you're using:
--quickpkg-direct=y (if I don't have binary packages yet)
With the --usepkgonly and --ignore-soname-deps=n options, the libgcc_s.so.1 should be pulled in automatically as an soname dependency.