v8: fix v3 builds by switching to GCC 5 (mksnapshot segfaults with GCC 6)

pkgs/top-level/all-packages.nix

···
   v8_3_16_14 = callPackage ../development/libraries/v8/3.16.14.nix {
     inherit (python2Packages) python gyp;
     cctools = darwin.cctools;
+    stdenv = overrideCC stdenv gcc5;
   };
 
   v8_3_24_10 = callPackage ../development/libraries/v8/3.24.10.nix {
     inherit (python2Packages) python gyp;
+    stdenv = overrideCC stdenv gcc5;
   };
 
   v8 = callPackage ../development/libraries/v8 {