Allows cmake to find 32-bit DB2 on Windows x64.
Fixes test_prepared_insert_with_orm_type with DB2.

DB2 backend seems to follow the implementation ODBC one, so this fix is almost a copy of odbc_standard_use_type_backend.
db2_standard_use_type_backend calculates the size of variables during bind() while ODBC does it during pre_use().
If the value (strings specifically) has its size changed between bind (statement::prepare) and pre_use (statement::execute) the backend will have the previous size.
This difference results in wrongly sized buffers what may lead to memory corruption.