Use emplace_back() in TB code

The patch was tested for correctness by running bench with and
without the change against current master, and the tablebase hit
numbers were found to be identical in both cases. See the pull
request comments for details:
#1826

No functional change.

src/syzygy/tbprobe.cpp

@@ -1277,7 +1277,7 @@ void Tablebases::init(const std::string& paths) {
                         continue; // First on diagonal, second above
 
                     else if (!off_A1H8(s1) && !off_A1H8(s2))
-                        bothOnDiagonal.push_back(std::make_pair(idx, s2));
+                        bothOnDiagonal.emplace_back(idx, s2);
 
                     else
                         MapKK[idx][s2] = code++;

src/tt.cpp

@@ -84,7 +84,7 @@ void TranspositionTable::clear() {
 
   for (size_t idx = 0; idx < Options["Threads"]; idx++)
   {
-      threads.push_back(std::thread([this, idx]() {
+      threads.emplace_back([this, idx]() {
 
           // Thread binding gives faster search on systems with a first-touch policy
           if (Options["Threads"] > 8)
@@ -97,7 +97,7 @@ void TranspositionTable::clear() {
                                 stride : clusterCount - start;
 
           std::memset(&table[start], 0, len * sizeof(Cluster));
-      }));
+      });
   }
 
   for (std::thread& th: threads)