make sure we store the function pointers to the real push/pop functions rather than...

make sure we store the function pointers to the real push/pop functions rather than the public weak ones so that the libc->libpthread forwarding code is able to work properly

this should fix the case where libpthread.so is not linked directly, but rather via another library:
 app -> links to libfoo.so -> links to libpthread.so

and any function (like readdir_r) that does:
 __UCLIBC_MUTEX_LOCK()
 __UCLIBC_MUTEX_UNLOCK()