Revert __HAVE_64B_ATOMICS configure check

The 53807741fb added a configure check
for 64-bit atomic operations that were not previously enabled on some
32-bit ABIs.

However, the NPTL semaphore code casts a sem_t to a new_sem and issues
a 64-bit atomic operation for __HAVE_64B_ATOMICS.  Since sem_t has
32-bit alignment on 32-bit architectures, this prevents the use of
64-bit atomics even if the ABI supports them.

Assume 64-bit atomic support from __WORDSIZE, which maps to how glibc
defines it before the broken change.  Also rename __HAVE_64B_ATOMICS
to USE_64B_ATOMICS to define better the flag meaning.

Checked on x86_64-linux-gnu and i686-linux-gnu.

Reviewed-by: Wilco Dijkstra  <Wilco.Dijkstra@arm.com>

nptl/sem_init.c

@@ -38,7 +38,7 @@ __new_sem_init (sem_t *sem, int pshared, unsigned int value)
   struct new_sem *isem = (struct new_sem *) sem;
 
   /* Use the values the caller provided.  */
-#if __HAVE_64B_ATOMICS
+#if USE_64B_ATOMICS
   isem->data = value;
 #else
   isem->value = value << SEM_VALUE_SHIFT;