gcc6, gcc7, gcc8, gcc9, gcc10, gcc11: do not modify code in prePatch

Doing any modifications before patching risks invalidating patch base.

pkgs/development/compilers/gcc/8/default.nix

@@ -101,9 +101,15 @@ stdenv.mkDerivation ({
 
   hardeningDisable = [ "format" "pie" ];
 
+  postPatch = ''
+    configureScripts=$(find . -name configure)
+    for configureScript in $configureScripts; do
+      patchShebangs $configureScript
+    done
+  ''
   # This should kill all the stdinc frameworks that gcc and friends like to
   # insert into default search paths.
-  prePatch = lib.optionalString hostPlatform.isDarwin ''
+  + lib.optionalString hostPlatform.isDarwin ''
     substituteInPlace gcc/config/darwin-c.c \
       --replace 'if (stdinc)' 'if (0)'
 
@@ -112,14 +118,8 @@ stdenv.mkDerivation ({
 
     substituteInPlace libgfortran/configure \
       --replace "-install_name \\\$rpath/\\\$soname" "-install_name ''${!outputLib}/lib/\\\$soname"
-  '';
-
-  postPatch = ''
-    configureScripts=$(find . -name configure)
-    for configureScript in $configureScripts; do
-      patchShebangs $configureScript
-    done
-  '' + (
+  ''
+  + (
     if targetPlatform != hostPlatform || stdenv.cc.libc != null then
       # On NixOS, use the right path to the dynamic linker instead of
       # `/lib/ld*.so'.