stdenv bootstrap tools: adjust with glibc-2.34

https://hydra.nixos.org/build/171494041

pkgs/stdenv/linux/bootstrap-tools/scripts/unpack-bootstrap-tools.sh

@@ -29,8 +29,9 @@ for i in $out/bin/* $out/libexec/gcc/*/*/*; do
         ./patchelf --set-interpreter $LD_BINARY --set-rpath $out/lib --force-rpath "$i"
 done
 
+# With glibc-2.34 the librt so-file is named a bit differently and doesn't need patchelfing.
 for i in $out/lib/librt-*.so $out/lib/libpcre*; do
-    if [ -L "$i" ]; then continue; fi
+    if [ -L "$i" ] || [ ! -e "$i" ]; then continue; fi
     echo patching "$i"
     $out/bin/patchelf --set-rpath $out/lib --force-rpath "$i"
 done