Set 2GB TT size limit for 32 bits

Import this simplified version
that has been committed upstream.

Now TT code is the same of official.

No functional change.

src/tt.cpp

@@ -30,11 +30,10 @@ TranspositionTable TT; // Our global transposition table
 /// measured in megabytes. Transposition table consists of a power of 2 number
 /// of clusters and each cluster consists of TTClusterSize number of TTEntry.
 
-void TranspositionTable::resize(uint64_t mbSize) {
+void TranspositionTable::resize(size_t mbSize) {
 
   assert(sizeof(Cluster) == CacheLineSize / 2);
 
-  // Here mbSize should be 64 bit to not overflow (mbSize * 1024 * 1024)
   size_t newClusterCount = size_t(1) << msb((mbSize * 1024 * 1024) / sizeof(Cluster));
 
   if (newClusterCount == clusterCount)

src/tt.h

@@ -86,7 +86,7 @@ class TranspositionTable {
   void new_search() { generation8 += 4; } // Lower 2 bits are used by Bound
   uint8_t generation() const { return generation8; }
   TTEntry* probe(const Key key, bool& found) const;
-  void resize(uint64_t mbSize);
+  void resize(size_t mbSize);
   void clear();
 
   // The lowest order bits of the key are used to get the index of the cluster

src/ucioption.cpp

@@ -52,7 +52,7 @@ bool CaseInsensitiveLess::operator() (const string& s1, const string& s2) const
 
 void init(OptionsMap& o) {
 
-  const int MaxHashMB = Is64Bit ? 1024 * 1024 : 65536;
+  const int MaxHashMB = Is64Bit ? 1024 * 1024 : 2048;
 
   o["Write Debug Log"]       << Option(false, on_logger);
   o["Contempt"]              << Option(0, -100, 100);