[darwin.stdenv] Fix to work on Sierra

This reinstates the libSystem selective symbol export machinery we used
to have, but locks it to the symbols that were present in 10.11 and skips
the actual compiled code we put into that library in favor of the system
initialization code. That should make it more stable and less likely to
do weird stuff than the last time we did this.

pkgs/stdenv/darwin/unpack-bootstrap-tools.sh

@@ -17,9 +17,44 @@ for i in $out/bin/*; do
   fi
 done
 
+install_name_tool \
+  -id $out/lib/system/libsystem_c.dylib \
+  $out/lib/system/libsystem_c.dylib
+
+install_name_tool \
+  -id $out/lib/system/libsystem_kernel.dylib \
+  $out/lib/system/libsystem_kernel.dylib
+
+# TODO: this logic basically duplicates similar logic in the Libsystem expression. Deduplicate them!
+libs=$(otool -arch x86_64 -L /usr/lib/libSystem.dylib | tail -n +3 | awk '{ print $1 }')
+
+for i in $libs; do
+  if [ "$i" != "/usr/lib/system/libsystem_kernel.dylib" ] && [ "$i" != "/usr/lib/system/libsystem_c.dylib" ]; then
+    args="$args -reexport_library $i"
+  fi
+done
+
+ld -macosx_version_min 10.7 \
+   -arch x86_64 \
+   -dylib \
+   -o $out/lib/libSystem.B.dylib \
+   -compatibility_version 1.0 \
+   -current_version 1226.10.1 \
+   -reexport_library $out/lib/system/libsystem_c.dylib \
+   -reexport_library $out/lib/system/libsystem_kernel.dylib \
+   $args
+
+ln -s libSystem.B.dylib $out/lib/libSystem.dylib
+
+for name in c dbm dl info m mx poll proc pthread rpcsvc util gcc_s.10.4 gcc_s.10.5; do
+  ln -s libSystem.dylib $out/lib/lib$name.dylib
+done
+
+ln -s libresolv.9.dylib $out/lib/libresolv.dylib
+
 for i in $out/lib/*.dylib $out/Library/Frameworks/CoreFoundation.framework/Versions/A/CoreFoundation; do
-  if ! test -L $i; then
-    echo patching $i
+  if test ! -L "$i" -a "$i" != "$out/lib/libSystem*.dylib"; then
+    echo "Patching $i"
 
     id=$(otool -D "$i" | tail -n 1)
     install_name_tool -id "$(dirname $i)/$(basename $id)" $i