Makefile: fix thread libraries stripping

Strip libthread_db the same as any other library, but strip libpthread
with --strip-debug. See the relevant mailing list discussion [1] for
additional details.

[1] http://lists.busybox.net/pipermail/buildroot/2012-October/060126.html



Signed-off-by: Richard Braun <rbraun@sceen.net>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>