nptl: Move nanosleep implementation to libc

Checked on x86_64-linux-gnu and powerpc64le-linux-gnu.  I also checked
the libpthread.so .gnu.version_d entries for every ABI affected and
all of them contains the required versions (including for architectures
which exports __nanosleep with a different version).

Reviewed-by: Florian Weimer <fweimer@redhat.com>