pkgs/top-level/stage: force elfv2 on static powerpc64-linux

The staging logic reconstructs the target platform, discarding
powerpc64's custom gcc.abi = elfv2 setup. Musl requires ELFv2 ABI so
this should be set unconditionally here.

pkgs/top-level/stage.nix

@@ -227,6 +227,8 @@ let
           }.${stdenv.hostPlatform.parsed.abi.name}
             or lib.systems.parse.abis.musl;
         };
+      } // lib.optionalAttrs (stdenv.hostPlatform.system == "powerpc64-linux") {
+        gcc.abi = "elfv2";
       };
     });
   };