Add 64-bit atomic ops for armv6.  The only safe way to access a 64-bit
value shared across multiple cores is with atomic_load_64() and
atomic_store_64(), because the normal 64-bit load/store instructions
are not atomic on 32-bit arm.  Luckily the ldrexd/strexd instructions
that are atomic are fairly cheap on armv6.  Because it's fairly simple
to do, this implements all the ops for 64-bit, not just load/store.

Reviewed by:	andrew, cognet
Split gethrtime() and gethrtime_waitfree() and make the former use
nanouptime() instead of getnanouptime().  nanouptime(9) provides more
precise result at expense of being slower.

In r269223, gethrtime() is used as creation time of dbuf, which in turn
acts as portion of lookup key to maintain AVL invariant where there can
not be duplicate items.  Before this change, gethrtime() have preferred
better execution time by sacrificing precision, which may lead to panic
on busy systems with:

	panic: avl_find() succeeded inside avl_add()

Reported by:	allanjude, mav
PR:		kern/192284
MFC after:	11 days
X-MFC-with:	r269223