lib: Clean up how linux and gcc config is specified

Second attempt of 8929989614589ee3acd070a6409b2b9700c92d65; see that
commit for details.

This reverts commit 0bc275e634.

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

@@ -1,7 +1,7 @@
 { lib, targetPlatform }:
 
 let
-  p =  targetPlatform.platform.gcc or {}
+  p =  targetPlatform.gcc or {}
     // targetPlatform.parsed.abi;
 in lib.concatLists [
   (lib.optional (!targetPlatform.isx86_64 && p ? arch) "--with-arch=${p.arch}") # --with-arch= is unknown flag on x86_64

pkgs/development/compilers/julia/1.0.nix

@@ -123,7 +123,7 @@ stdenv.mkDerivation rec {
     let
       arch = stdenv.lib.head (stdenv.lib.splitString "-" stdenv.system);
       march = {
-        x86_64 = stdenv.hostPlatform.platform.gcc.arch or "x86-64";
+        x86_64 = stdenv.hostPlatform.gcc.arch or "x86-64";
         i686 = "pentium4";
         aarch64 = "armv8-a";
       }.${arch}

pkgs/development/compilers/julia/1.3.nix

@@ -73,7 +73,7 @@ stdenv.mkDerivation rec {
     let
       arch = head (splitString "-" stdenv.system);
       march = {
-        x86_64 = stdenv.hostPlatform.platform.gcc.arch or "x86-64";
+        x86_64 = stdenv.hostPlatform.gcc.arch or "x86-64";
         i686 = "pentium4";
         aarch64 = "armv8-a";
       }.${arch}

pkgs/development/compilers/julia/1.5.nix

@@ -71,7 +71,7 @@ stdenv.mkDerivation rec {
     let
       arch = head (splitString "-" stdenv.system);
       march = {
-        x86_64 = stdenv.hostPlatform.platform.gcc.arch or "x86-64";
+        x86_64 = stdenv.hostPlatform.gcc.arch or "x86-64";
         i686 = "pentium4";
         aarch64 = "armv8-a";
       }.${arch}