gcc: link $lib/lib -> $lib/$targetConfig correctly and consistently

When native-compiling, gcc will install libraries into:

/nix/store/...-$targetConfig-gcc-$version-lib/lib

When cross-compiling, gcc will install libraries into:

/nix/store/...-$targetConfig-gcc-$version-lib/$targetConfig

When cross-compiling, we intended to create a link from $lib/lib to
$lib/$targetConfig, so that downstream users can always safely
assume that "${lib.getLib stdenv.cc.cc}/lib" is where the gcc
libraries are, regardless of whether `stdenv.cc.cc` is a cross
compiler or a native compiler.

Unfortunately, there were two problems with how we were trying to
create these links:

1. The link would be created only when `enableLibGccOutput==true`

2. The link was being created from the incorrect source
`$lib/lib/lib` instead of `$lib/lib`.

Both of these mistakes are my fault. This commit corrects them by
creating the link using `ln -Ts` (which is more predictable) and by
creating the link from `gcc/common/builder.nix` rather than from
`gcc/common/libgcc.nix`.

pkgs/development/compilers/gcc/common/builder.nix

···
 { lib
 , stdenv
 , enableMultilib
+, targetConfig
 }:
 
 let
···
   preInstall = ''
     mkdir -p "$out/''${targetConfig}/lib"
     mkdir -p "''${!outputLib}/''${targetConfig}/lib"
+  '' +
+  # if cross-compiling, link from $lib/lib to $lib/${targetConfig}.
+  # since native-compiles have $lib/lib as a directory (not a
+  # symlink), this ensures that in every case we can assume that
+  # $lib/lib contains the .so files
+  lib.optionalString (with stdenv; targetPlatform.config != hostPlatform.config) ''
+    ln -Ts "''${!outputLib}/''${targetConfig}/lib" $lib/lib
   '' +
   # Make `lib64` symlinks to `lib`.
   lib.optionalString (!enableMultilib && stdenv.hostPlatform.is64bit && !stdenv.hostPlatform.isMips64n32) ''

pkgs/development/compilers/gcc/common/libgcc.nix

···
     lib.optionalString (!langC) ''
       rm -f $out/lib/libgcc_s.so*
     ''
-    + lib.optionalString (hostPlatform != targetPlatform) ''
-      mkdir -p $lib/lib/
-      ln -s ${targetPlatformSlash}lib $lib/lib
-    ''
 
     # TODO(amjoseph): remove the `libgcc_s.so` symlinks below and replace them
     # with a `-L${gccForLibs.libgcc}/lib` in cc-wrapper's

pkgs/development/compilers/gcc/default.nix

···
     disableBootstrap = atLeast11 && !stdenv.hostPlatform.isDarwin && (atLeast12 -> !profiledCompiler);
 
     inherit (stdenv) buildPlatform hostPlatform targetPlatform;
+    targetConfig = if targetPlatform != hostPlatform then targetPlatform.config else null;
 
     patches = callFile ./patches {};
 
···
         buildPlatform
         hostPlatform
         targetPlatform
+        targetConfig
         patches
         crossMingw
         stageNameAddon
···
     ++ optional (is7 && targetPlatform.isAarch64) "--enable-fix-cortex-a53-843419"
     ++ optional (is7 && targetPlatform.isNetBSD) "--disable-libcilkrts";
 
-  targetConfig = if targetPlatform != hostPlatform then targetPlatform.config else null;
+  inherit targetConfig;
 
   buildFlags =
     # we do not yet have Nix-driven profiling