RandomX: added parameter for scratchpad prefetch mode

`scratchpad_prefetch_mode` can have 4 values:
0: off
1: use `prefetcht0` instruction (default, same as previous XMRig versions)
2: use `prefetchnta` instruction (faster on Coffee Lake and a few other CPUs)
3: use `mov` instruction

src/config.json

@@ -21,7 +21,8 @@
         "rdmsr": true,
         "wrmsr": true,
         "cache_qos": false,
-        "numa": true
+        "numa": true,
+        "scratchpad_prefetch_mode": 1
     },
     "cpu": {
         "enabled": true,