emulated atomic64: disable interrupts as the lock mechanism on !SMP

[jhibbits]

sys/kern/subr_atomic64.c

78

Why not make LOCK_A64() be more like:

{ register s; s = intr_disable();

and UNLOCK_A64() be:

intr_restore(s); }

Then you don't need the added LOCK_DECL.

I think pthread_cleanup_push() and pthread_cleanup_pop() do something like this.

[kevans]

Use better construct to eliminate separate LOCK_DECL that's only used for !SMP

[jhibbits]

LGTM

[bdragon]

The changes look good to me as well.