toolchain: common: iterable sections: add alternate foreach

Previously, it was possible to declare an iterable section using
the alternate form (where the `out_type` does not correspond
to the `struct_type`) but there was no convenient means to
iterate over them.

Add `STRUCT_SECTION_FOREACH_ALTERNATE()` to fill the gap.

Signed-off-by: Chris Friedt <cfriedt@meta.com>