glibc: copy libgcc_s.so from .lib output if it exists

Otherwise copy it from the default output. The difference is visible
when we build `glibc` with:

- `bootstrapTools` `gcc`: ${stdenv.cc.cc.out}/lib/libgcc_s.so.1 is used
- `nixpkgs` `gcc`: ${stdenv.cc.cc.lib}/lib/libgcc_s.so.1 is used

Noticed when experimented with multiple `gcc` rebuilds in bootstrap.

While at it killing `RUNPATH` reference to bootstrap `glibc`. We expect
`libgcc_s.so.1` to be used with currently installed `glibc` anyway.

pkgs/development/libraries/glibc/default.nix

@@ -75,11 +75,14 @@ callPackage ./common.nix { inherit stdenv; } {
     # - clang-wrapper in cross-compilation
     # Last attempt: https://github.com/NixOS/nixpkgs/pull/36948
     preInstall = ''
-      if [ -f ${stdenv.cc.cc}/lib/libgcc_s.so.1 ]; then
+      if [ -f ${lib.getLib stdenv.cc.cc}/lib/libgcc_s.so.1 ]; then
           mkdir -p $out/lib
-          cp ${stdenv.cc.cc}/lib/libgcc_s.so.1 $out/lib/libgcc_s.so.1
+          cp ${lib.getLib stdenv.cc.cc}/lib/libgcc_s.so.1 $out/lib/libgcc_s.so.1
           # the .so It used to be a symlink, but now it is a script
-          cp -a ${stdenv.cc.cc}/lib/libgcc_s.so $out/lib/libgcc_s.so
+          cp -a ${lib.getLib stdenv.cc.cc}/lib/libgcc_s.so $out/lib/libgcc_s.so
+          # wipe out reference to previous libc it was built against
+          chmod +w $out/lib/libgcc_s.so.1
+          patchelf --set-rpath $out/lib $out/lib/libgcc_s.so.1
       fi
     '';