Revert __HAVE_64B_ATOMICS configure check

Revert __HAVE_64B_ATOMICS configure check

The 53807741fb44edb8e7c094cb5e7d4ff4e92a6ec1 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>