gcc48: disable on x86_64-darwin

According to Hydra, the last time GCC 4.8 successfully built on Darwin
was October 2014. It is possible to make the first stage build
successfully with clang, but the resulting GCC is not capable of reading
the Darwin SDK headers due to their use of `__can_include`.

It’s been broken for almost a decade, so just disable it.

pkgs/development/compilers/gcc/all.nix

···
               else            /* "4.8" */   isl_0_14;
       } // lib.optionalAttrs (majorMinorVersion == "4.8") {
         texinfo = texinfo5; # doesn't validate since 6.1 -> 6.3 bump
-      } // lib.optionalAttrs (majorMinorVersion == "4.9") {
-        # Build fails on Darwin with clang
-        stdenv = if stdenv.isDarwin then gccStdenv else stdenv;
       } // lib.optionalAttrs (!(atLeast "6")) {
         cloog = if stdenv.isDarwin
                 then null

pkgs/development/compilers/gcc/default.nix

···
       maintainers
     ;
   } // lib.optionalAttrs (!atLeast11) {
-    badPlatforms = if !is49 then [ "aarch64-darwin" ] else lib.platforms.darwin;
+    badPlatforms = if !(is48 || is49) then [ "aarch64-darwin" ] else lib.platforms.darwin;
   };
 } // optionalAttrs is7 {
   env.NIX_CFLAGS_COMPILE = lib.optionalString (stdenv.cc.isClang && langFortran) "-Wno-unused-command-line-argument";