Move prefetch() out of TT

This code is platform specific and has nothing to
do with TT class, so move to misc.cpp

This patch is a prerequisite to use extend prefetch use
also to other hash tables apart from Transposition Table.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

src/misc.h

@@ -55,6 +55,7 @@ extern const std::string engine_name();
 extern int get_system_time();
 extern int cpu_count();
 extern int Bioskey();
+extern void prefetch(char* addr);
 
 
 ////