Replace log2 by implementing Log2 in options_builder

Summary:
log2 function is only used in options_builder, and this function
is not available under certain platform such as android.
This patch implements Log2 by log(n) / log(2).

Test Plan:
make

util/options_builder.cc

@@ -11,6 +11,10 @@ namespace rocksdb {
 
 namespace {
 
+double Log2(double n) {
+  return log(n) / log(2);
+}
+
 // For now, always use 1-0 as level bytes multiplier.
 const int kBytesForLevelMultiplier = 10;
 const size_t kBytesForOneMb = 1024 * 1024;
@@ -28,7 +32,7 @@ CompactionStyle PickCompactionStyle(size_t write_buffer_size,
       ::log(target_db_size / write_buffer_size) / ::log(kBytesForLevelMultiplier)));
 
   int expected_max_files_universal =
-      static_cast<int>(ceil(log2(target_db_size / write_buffer_size)));
+      static_cast<int>(ceil(Log2(target_db_size / write_buffer_size)));
 
   const int kEstimatedLevel0FilesInLevelStyle = 2;
   // Estimate write amplification: