gcc: support -m32 on ppc64le

This is needed to build grub2 for powerpc64le hosts. Running powerpcle code
on powerpc64le is somewhat analogous to running multiarch i686 code on x86_64,
so it's also useful to have in general.

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

@@ -147,6 +147,10 @@ let
       (lib.enableFeature enablePlugin "plugin")
     ]
 
+    # Support -m32 on powerpc64le
+    ++ lib.optional (targetPlatform.system == "powerpc64le-linux")
+      "--enable-targets=powerpcle-linux"
+
     # Optional features
     ++ lib.optional (isl != null) "--with-isl=${isl}"
     ++ lib.optionals (cloog != null) [