autoPatchelfHook: Patch PIC exes/libs as well

If there is a shared object or executable that's using
position-independent code, the file's mime type is
"application/x-pie-executable", so until this change its dependencies
wouldn't be patched.

This simply adds the mime type to the search loop.

Signed-off-by: aszlig <aszlig@nix.build>
(cherry picked from commit ff5cecf)
Reason: The fix is non-intrusive and should not break anything that
        wasn't broken before. I've tested whether oracle-instantclient
        builds and it still does. Other than that no other package is
        using autoPatchelfHook in NixOS 18.03.

pkgs/build-support/setup-hooks/auto-patchelf.sh

@@ -15,6 +15,7 @@ findElfs() {
         while [ -n "$1" ]; do
             mimeType="$(file -b -N --mime-type "$1")"
             if [ "$mimeType" = application/x-executable \
+              -o "$mimeType" = application/x-pie-executable \
               -o "$mimeType" = application/x-sharedlib ]; then
                 echo "$1"
             fi