[RFC] ppc64le enablement (#45340)

* ppc64le enablement

* gcc, glibc: properly handle __float128

* lib/systems, stdenv: syntax cleanup

* gcc7: remove ugly hack

* gcc: add/update __float128 flags

* stdenv: add another pair of quotes for consistency

* gcc: move __float128 flag for ppc64le-glibc into common/platform-flags.nix