scripts: gen_relocate_app.py: fix undefined _ADDR

With the addition of #34185, it is not longer gauranteed that for a
memory region `NAME` there exists a symbol `NAME_ADDR`. Use the linker
builtin function `ORIGIN` to instead directly get the start address of
the selected memory region.

Signed-off-by: Jordan Yates <jordan.yates@data61.csiro.au>