native_exec regression: dr_app_running_under_dynamorio is marked as a gateway

Originally is_native_pc() only checked native_exec_areas. But 6a8d7d2a added "is_dr_native_pc() ||" to include DR routines.

The problem is that is_native_pc is also used for at_native_exec_gateway(). If native_exec_areas is non-empty, it then considers dr_app_running_under_dynamorio to be a place where we have to go native.

The query routine needs to be split: the native_elf calls should include DR entries, but gateway checks should not.