libc/libthread_xu - Fix segfault caused by libpthread used before initialised

libc/libthread_xu - Fix segfault caused by libpthread used before initialised

If a binary has libpthread as dependency and is linked against libintl,
or any other library that uses pthread locking primitives from libc
without depending on libpthread itself it can happen that functions
from libpthread are called before libpthread's constructor has been
called. This leads to a segfault.

This fix adds a weak symbold to libc, namely _pthread_init_early,
which is called from a libc constructor function and therefore makes
sure that libpthread is initialised early enough.

Thanks to Simon Schubert, Angelos Oikonomopoulos, and Sascha Wildner
for the support in debugging this.