atomic: Reinstate HAVE_64B_ATOMICS configure check

atomic: Reinstate HAVE_64B_ATOMICS configure check

Reinstate HAVE_64B_ATOMICS configure check that was reverted by commit
7fec8a5de6826ef9ae440238d698f0fe5a5fb372 due to BZ #33632.  This was
fixed by 3dd2cbfa35e0e6e0345633079bd5a83bb822c2d8 by only allowing
64-bit atomics on sem_t if its type is 8-byte aligned.  Rebase and add
in cleanups in include/atomic.h that were omitted.

Fix an issue with sparcv8-linux-gnu-leon3 forcing -mcpu=v8 for rtld.c which
overrules -mcpu=leon3 and causes __atomic_always_lock_free (4, 0) to
incorrectly return 0 and trigger asserts in atomics.  Remove this as it
seems to be a workaround for an issue in 1997.

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