Don't use __builtin_eh_return_data_regno for SjLj

This also gets rid of RAISE_EXCEPTION and instead defines
_Unwind_RaiseException to _Unwind_SjLj_RaiseException for SjLj.

Additionally, it moves the defines to the top of the file.
Only use LIBOBJFW_DEP after including buildsys.mk

Otherwise, it does not work properly.