More aggsum optimizations

- Avoid atomic_add() when updating as_lower_bound/as_upper_bound.
Previous code was excessively strong on 64bit systems while not
strong enough on 32bit ones.  Instead introduce and use real
atomic_load() and atomic_store() operations, just an assignments
on 64bit machines, but using proper atomics on 32bit ones to avoid
torn reads/writes.

 - Reduce number of buckets on large systems.  Extra buckets not as
much improve add speed, as hurt reads.  Unlike wmsum for aggsum
reads are still important.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored-By: iXsystems, Inc.
Closes #12145

lib/libspl/asm-generic/atomic.c

@@ -390,6 +390,19 @@ atomic_swap_ptr(volatile void *target, void *bits)
 	return (old);
 }
 
+#ifndef _LP64
+uint64_t
+atomic_load_64(volatile uint64_t *target)
+{
+	return (__atomic_load_n(target, __ATOMIC_RELAXED));
+}
+
+void
+atomic_store_64(volatile uint64_t *target, uint64_t bits)
+{
+	return (__atomic_store_n(target, bits, __ATOMIC_RELAXED));
+}
+#endif
 
 int
 atomic_set_long_excl(volatile ulong_t *target, uint_t value)