[AMDGPU] Emit backend_stack_size PAL metadata (#72509)

For chain functions, PAL uses a `backend_stack_size` metadata item,
which at the moment has the same meaning as `stack_frame_size_in_bytes`.
We emit both for now in order to simplify coordination with PAL.

The new item must be emitted in the `shader_functions` section, just as
the metadata for other module entry functions. For simplicity, we mark
chain functions as module entry functions and emit the same metadata for
all of them.