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/core/config/Config_default.h

@@ -51,7 +51,12 @@ R"===(
     "randomx": {
         "init": -1,
         "mode": "auto",
-        "numa": true
+        "1gb-pages": false,
+        "rdmsr": true,
+        "wrmsr": true,
+        "cache_qos": false,
+        "numa": true,
+        "scratchpad_prefetch_mode": 1
     },
     "cpu": {
         "enabled": true,