orctarget: Fix default target selection not applying when retrieving it by name

Hi,

This MR is to fix an edge case of the environment variable-based target selection. If someone queries a target by name at runtime, if not found, it should fall back to the default case. But this was not delegated to orc_target_get_default() in this particular function, so any test that used ORC_BACKEND and explicitly passed a NULL target name to the compiler would instead be building for AVX and not the specified backend.