linuxthreads.old: only declare __pthread_xxx funcs hidden inside of libpthread

linuxthreads.old: only declare __pthread_xxx funcs hidden inside of libpthread

These stubs get used in libc as forwarding funcs, so we don't want them
labeled as hidden.  Otherwise, attempting to use them in other libraries
(like libdl) will result in references to symbols that only libpthread
provides.  Such as:
	/usr/bin/python2.6: can't resolve symbol '__pthread_mutex_lock' in lib '/lib/libdl.so.0'.

This also brings the old code in line with the newer linuxthreads.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>