Fix alignment in relaxed atomics spec test (#8961)

The spec test intends to use the natural alignment for all instructions,
but it's choosing an alignment based on the memory address type rather
than the size of the load/store. This means that sometimes the alignment
is too small (4 bytes for an 8-byte load/store in a 32-bit memory), and
sometimes the alignment is too large (8 bytes for a 4-byte load/store in
a 64-bit memory) both of which are a [validation
error](https://github.com/stevenfontanella/threads/blob/f521d7b3c19bcc9097aad979b3b91837eab88d83/interpreter/valid/valid.ml#L206)
(but are not in Binaryen today).

Use the size of the load/store as the alignment instead.

Spec reference:
<img width="737" height="216" alt="image"
src="https://github.com/user-attachments/assets/b86de03e-1316-4c2c-9a12-80773fb88119"
/>