MYSQL_PLUGIN_IMPORT did not work correctly for the RocksDB helper library
rocksdb_aux_lib, because that library was not compiled with
-DMYSQL_DYNAMIC_PLUGIN.

Fix DBUG such that it does not depend on exported data, only on functions
(which do not need MYSQL_PLUGIN_IMPORT decoration)

Use a "getter" function _db_my_assert() instead of DLL-exported variable.

Also, reduce object code duplication by moving more of the DBUG_ASSERT
logic inside the _db_my_assert() function, and add unlikely() and
ATTRIBUTE_COLD hints to ensure that the 'assertion failed' code will
be separated from the main control flow logic. Thus, the compiler can
move the unlikely() code to the end of the compiled function, reachable
via a forward conditional branch, which the processor's branch predictor
could assume 'not taken'.