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