string: Implement strerror in terms of strerror_l

If the thread is terminated then __libc_thread_freeres will free the
storage via __glibc_tls_internal_free.

It is only within the calling thread that this matters.  It makes
strerror MT-safe.

Checked on x86-64-linux-gnu, i686-linux-gnu, powerpc64le-linux-gnu,
and s390x-linux-gnu.

Tested-by: Carlos O'Donell <carlos@redhat.com>
Reviewed-by: Carlos O'Donell <carlos@redhat.com>