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/randomx/randomx.h

@@ -200,6 +200,8 @@ void randomx_apply_config(const T& config)
 	RandomX_CurrentConfig.Apply();
 }
 
+void randomx_set_scratchpad_prefetch_mode(int mode);
+
 #if defined(__cplusplus)
 extern "C" {
 #endif