[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

pkgs/development/compilers/gcc/common/platform-flags.nix

@@ -10,4 +10,7 @@ in lib.concatLists [
   (lib.optional (p ? fpu) "--with-fpu=${p.fpu}")
   (lib.optional (p ? float) "--with-float=${p.float}")
   (lib.optional (p ? mode) "--with-mode=${p.mode}")
+  (lib.optional
+    (let tp = targetPlatform; in tp.isPower && tp.libc == "glibc" && tp.is64bit && tp.isLittleEndian)
+    "--with-long-double-128")
 ]