gcc_latest: add alias

Our gcc version often lacks behind the latest supported version.
To get the latest possible gcc version with all optimization/checks,
this RFC adds an gcc_latest alias.

pkgs/top-level/all-packages.nix

@@ -8475,6 +8475,8 @@ in
     isl = if !stdenv.isDarwin then isl_0_17 else null;
   }));
 
+  gcc_latest = gcc10;
+
   gfortran = gfortran9;
 
   gfortran48 = wrapCC (gcc48.cc.override {