nptl: Move __pthread_unwind_next into libc

It's necessary to stub out __libc_disable_asynccancel and
__libc_enable_asynccancel via rtld-stubbed-symbols because the new
direct references to the unwinder result in symbol conflicts when the
rtld exception handling from libc is linked in during the construction
of librtld.map.

unwind-forcedunwind.c is merged into unwind-resume.c.  libc now needs
the functions that were previously only used in libpthread.

The GLIBC_PRIVATE exports of __libc_longjmp and __libc_siglongjmp are
no longer needed, so switch them to hidden symbols.

The symbol __pthread_unwind_next has been moved using
scripts/move-symbol-to-libc.py.

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

nptl/pthread_cancel.c

@@ -23,6 +23,9 @@
 #include <atomic.h>
 #include <sysdep.h>
 #include <unistd.h>
+#include <unwind-link.h>
+#include <stdio.h>
+#include <gnu/lib-names.h>
 
 int
 __pthread_cancel (pthread_t th)
@@ -36,7 +39,12 @@ __pthread_cancel (pthread_t th)
 
 #ifdef SHARED
   /* Trigger an error if libgcc_s cannot be loaded.  */
-  __pthread_unwind_link_get ();
+  {
+    struct unwind_link *unwind_link = __libc_unwind_link_get ();
+    if (unwind_link == NULL)
+      __libc_fatal (LIBGCC_S_SO
+		    " must be installed for pthread_cancel to work\n");
+  }
 #endif
   int result = 0;
   int oldval;