stdenv: In the fixupPhase, make the output writable first

Cherry-picked from 1d11c1dc48 in the
multiple-outputs branch.

pkgs/stdenv/generic/setup.sh

@@ -288,9 +288,6 @@ stripDirs() {
 
     if [ -n "${dirs}" ]; then
         header "stripping (with flags $stripFlags) in $dirs"
-        for dir in $dirs; do
-            test -L "$dir" || chmod -R +rw "$dir"
-        done
         find $dirs -type f -print0 | xargs -0 ${xargsFlags:--r} strip $commonStripFlags $stripFlags || true
         stopNest
     fi
@@ -735,6 +732,9 @@ installPhase() {
 fixupPhase() {
     runHook preFixup
 
+    # Make sure everything is writable so "strip" et al. work.
+    chmod -R u+w "$prefix"
+
     # Put man/doc/info under $out/share.
     forceShare=${forceShare:=man doc info}
     if [ -n "$forceShare" ]; then