nptl: Move pthread_getattr_default_np into libc

The symbol was moved using scripts/move-symbol-to-libc.py.

A new placeholder symbol __libpthread_version_placeholder@GLIBC_2.18
is needed to keep the GLIBC_2.18 symbol version in libpthread.
The __pthread_getattr_default_np@@GLIBC_PRIVATE export is used
from pthread_create.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>