Fix crashing under ARMv6

[XIELongDragon]

ristretto will crash under ARM32. It is caused by the function goLoad64 at

https://golang.org/src/runtime/internal/atomic/atomic_arm.go line 129

//go:nosplit
func goLoad64(addr *uint64) uint64 {
	if uintptr(unsafe.Pointer(addr))&7 != 0 {
		*(*int)(nil) = 0 // crash on unaligned uint64
	}
	_ = *addr // if nil, fault before taking the lock
	var r uint64
	addrLock(addr).lock()
	r = *addr
	addrLock(addr).unlock()
	return r
}

The root cause is the sampledLFU unaligned

---

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[XIELongDragon]

goroutine 22 [running]:
runtime/internal/atomic.goLoad64(0x19b1744, 0x182bf9c, 0x3)
        /usr/local/go/src/runtime/internal/atomic/atomic_arm.go:131 +0x1c
github.com/dgraph-io/ristretto.(*sampledLFU).getMaxCost(...)
        /home/pi/neurons/vendor/github.com/dgraph-io/ristretto/policy.go:296
github.com/dgraph-io/ristretto.(*defaultPolicy).Add(0x19b1760, 0x3c60b443, 0x0, 0x38, 0x0, 0x0, 0x0, 0x0, 0x0)
        /home/pi/neurons/vendor/github.com/dgraph-io/ristretto/policy.go:140 +0x80
github.com/dgraph-io/ristretto.(*Cache).processItems(0x191fc80)
        /home/pi/neurons/vendor/github.com/dgraph-io/ristretto/cache.go:435 +0x29c
created by github.com/dgraph-io/ristretto.NewCache
        /home/pi/neurons/vendor/github.com/dgraph-io/ristretto/cache.go:204 +0x32c

[XIELongDragon]

@jarifibrahim @manishrjain can you review please ?

[jarifibrahim]

@XIELongDragon apologies for the delay in review. Can you please add this comment https://github.com/dgraph-io/ristretto/pull/249/files#diff-2f3d176e1cb53549b532cb1313a54835140ac2ab5e3bbd6ea6b06f174ca5241dR282 ?

[XIELongDragon]

@XIELongDragon apologies for the delay in review. Can you please add this comment https://github.com/dgraph-io/ristretto/pull/249/files#diff-2f3d176e1cb53549b532cb1313a54835140ac2ab5e3bbd6ea6b06f174ca5241dR282 ?

@jarifibrahim comments added