gcc: provide both native and cross forms of gcc.libs libraries

I would like to use --sysroot=/nix/store/does/not/exist hack
for both `gcc` and `clang` drivers to remove default include
(and library) search paths when we override them with libc.

For `gcc` it works as is. But for `clang` it also drops some of
`gcc` search paths`. Let'sconsider 2 lookups.

Successful lookup (no `--sysroot`):

    $ printf "int main(){}" | clang++ -x c++ - -Wl,--verbose |& grep -F stdc++
    attempt to open /nix/store/...-gcc-11.3.0/lib/gcc/x86_64-unknown-linux-gnu/11.3.0/libstdc++.so failed
    attempt to open /nix/store/...-gcc-11.3.0-lib/x86_64-unknown-linux-gnu/lib/libstdc++.so failed
    attempt to open /nix/store/...-clang-11.1.0-lib/lib/libstdc++.so failed
    attempt to open /nix/store/...-gcc-11.3.0/lib64/gcc/x86_64-unknown-linux-gnu/11.3.0/libstdc++.so failed
    attempt to open /nix/store/...-gcc-11.3.0/lib64/gcc/x86_64-unknown-linux-gnu/11.3.0/../../../../lib64/libstdc++.so failed
    /nix/store/...-gcc-11.3.0/lib64/gcc/x86_64-unknown-linux-gnu/11.3.0/../../../../lib64/libstdc++.a

Failed lookup (has `--sysroot`):

    $ printf "int main(){}" | clang++ --sysroot=/does/not/exist -x c++ - -Wl,--verbose |& grep -F stdc++
    attempt to open /nix/store/...-gcc-11.3.0/lib/gcc/x86_64-unknown-linux-gnu/11.3.0/libstdc++.so failed
    attempt to open /nix/store/...-gcc-11.3.0-lib/x86_64-unknown-linux-gnu/lib/libstdc++.so failed
    attempt to open /nix/store/...-clang-11.1.0-lib/lib/libstdc++.so failed
    attempt to open /nix/store/...-gcc-11.3.0/lib64/gcc/x86_64-unknown-linux-gnu/11.3.0/libstdc++.so failed
    /nix/store/...-binutils-2.39/bin/ld: cannot find -lstdc++: No such file or directory

Note how `clang` starts the search roughly from
`gcc-11.3.0-lib/x86_64-unknown-linux-gnu/lib` in both cases. I think
it's our preferred location for both native and cross cases.

The change adds such a symlink:

    `gcc-11.3.0-lib` -> `gcc-11.3.0-lib/x86_64-unknown-linux-gnu`

pkgs/development/compilers/gcc/10/default.nix

@@ -222,6 +222,7 @@ stdenv.mkDerivation ({
   };
 
   targetConfig = if targetPlatform != hostPlatform then targetPlatform.config else null;
+  targetPlatformConfig = targetPlatform.config;
 
   buildFlags = optional
     (targetPlatform == hostPlatform && hostPlatform == buildPlatform)

pkgs/development/compilers/gcc/11/default.nix

@@ -226,6 +226,7 @@ stdenv.mkDerivation ({
   };
 
   targetConfig = if targetPlatform != hostPlatform then targetPlatform.config else null;
+  targetPlatformConfig = targetPlatform.config;
 
   buildFlags = optional
     (targetPlatform == hostPlatform && hostPlatform == buildPlatform)

pkgs/development/compilers/gcc/12/default.nix

@@ -264,6 +264,7 @@ stdenv.mkDerivation ({
   };
 
   targetConfig = if targetPlatform != hostPlatform then targetPlatform.config else null;
+  targetPlatformConfig = targetPlatform.config;
 
   buildFlags = optional
     (targetPlatform == hostPlatform && hostPlatform == buildPlatform)

pkgs/development/compilers/gcc/4.8/default.nix

@@ -227,6 +227,7 @@ stdenv.mkDerivation ({
   };
 
   targetConfig = if targetPlatform != hostPlatform then targetPlatform.config else null;
+  targetPlatformConfig = targetPlatform.config;
 
   buildFlags = optional
     (targetPlatform == hostPlatform && hostPlatform == buildPlatform)

pkgs/development/compilers/gcc/4.9/default.nix

@@ -247,6 +247,7 @@ stdenv.mkDerivation ({
   };
 
   targetConfig = if targetPlatform != hostPlatform then targetPlatform.config else null;
+  targetPlatformConfig = targetPlatform.config;
 
   buildFlags = optional
     (targetPlatform == hostPlatform && hostPlatform == buildPlatform)

pkgs/development/compilers/gcc/6/default.nix

@@ -258,6 +258,7 @@ stdenv.mkDerivation ({
   };
 
   targetConfig = if targetPlatform != hostPlatform then targetPlatform.config else null;
+  targetPlatformConfig = targetPlatform.config;
 
   buildFlags = optional
     (targetPlatform == hostPlatform && hostPlatform == buildPlatform)

pkgs/development/compilers/gcc/7/default.nix

@@ -226,6 +226,7 @@ stdenv.mkDerivation ({
   ;
 
   targetConfig = if targetPlatform != hostPlatform then targetPlatform.config else null;
+  targetPlatformConfig = targetPlatform.config;
 
   buildFlags = optional
     (targetPlatform == hostPlatform && hostPlatform == buildPlatform)

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

@@ -206,6 +206,7 @@ stdenv.mkDerivation ({
   };
 
   targetConfig = if targetPlatform != hostPlatform then targetPlatform.config else null;
+  targetPlatformConfig = targetPlatform.config;
 
   buildFlags = optional
     (targetPlatform == hostPlatform && hostPlatform == buildPlatform)

pkgs/development/compilers/gcc/9/default.nix

@@ -221,6 +221,7 @@ stdenv.mkDerivation ({
   };
 
   targetConfig = if targetPlatform != hostPlatform then targetPlatform.config else null;
+  targetPlatformConfig = targetPlatform.config;
 
   buildFlags = optional
     (targetPlatform == hostPlatform && hostPlatform == buildPlatform)

pkgs/development/compilers/gcc/builder.sh

@@ -203,6 +203,17 @@ preInstall() {
         ln -s lib "$out/${targetConfig}/lib32"
         ln -s lib "${!outputLib}/${targetConfig}/lib32"
     fi
+
+    # cc-wrappers uses --sysroot=/nix/store/does/not/exist as a way to
+    # drop default sysheaders search path. Unfortunately that switches
+    # clang++ into searching libraries in gcc in cross-compiler paths:
+    #   from ${!outputLib}/lib (native)
+    #   to ${!outputLib}/${targetPlatformConfig}/lib
+    # We create the symlink to make both native and cross paths
+    # available even if the toolchain is not the cross-compiler.
+    if [ ! -e ${!outputLib}/${targetPlatformConfig} ] ; then
+        ln -s . ${!outputLib}/${targetPlatformConfig}
+    fi
 }