string: Remove old TLS usage on strsignal

The per-thread state is refactored two use two strategies:

  1. The default one uses a TLS structure, which will be placed in the
     static TLS space (using __thread keyword).

  2. Linux allocates via struct pthread and access it through THREAD_*
     macros.

The default strategy has the disadvantage of increasing libc.so static
TLS consumption and thus decreasing the possible surplus used in
some scenarios (which might be mitigated by BZ#25051 fix).

It is used only on Hurd, where accessing the thread storage in the in
single thread case is not straightforward (afaiu, Hurd developers could
correct me here).

The fallback static allocation used for allocation failure is also
removed: defining its size is problematic without synchronizing with
translated messages (to avoid partial translation) and the resulting
usage is not thread-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>

sysdeps/unix/sysv/linux/tls-internal.c

@@ -0,0 +1 @@
+/* Empty.  */