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/crypto/rx/RxConfig.h

@@ -50,6 +50,14 @@ public:
         ModeMax
     };
 
+    enum ScratchpadPrefetchMode : uint32_t {
+        ScratchpadPrefetchOff,
+        ScratchpadPrefetchT0,
+        ScratchpadPrefetchNTA,
+        ScratchpadPrefetchMov,
+        ScratchpadPrefetchMax,
+    };
+
     bool read(const rapidjson::Value &value);
     rapidjson::Value toJSON(rapidjson::Document &doc) const;
 
@@ -68,6 +76,8 @@ public:
     inline bool cacheQoS() const        { return m_cacheQoS; }
     inline Mode mode() const            { return m_mode; }
 
+    inline ScratchpadPrefetchMode scratchpadPrefetchMode() const { return m_scratchpadPrefetchMode; }
+
 #   ifdef XMRIG_FEATURE_MSR
     const char *msrPresetName() const;
     const MsrItems &msrPreset() const;
@@ -94,6 +104,8 @@ private:
     int m_threads       = -1;
     Mode m_mode         = AutoMode;
 
+    ScratchpadPrefetchMode m_scratchpadPrefetchMode = ScratchpadPrefetchT0;
+
 #   ifdef XMRIG_FEATURE_HWLOC
     std::vector<uint32_t> m_nodeset;
 #   endif