u3: fix alignment in u3a_pile_prep

[yosoyubik]

No description provided.

[joemfb]

I don't think we want or need to change the frame size to accomplish alignment. The frame size (in words) is the sizeof (in bytes), so it already accounts for any padding necessary to align an array of these frames. What's missing is the initial alignment of the base pointer (ie, cap_p). That needs to happen, and the result stored in top_p. The pile also needs an additional member to track and restore (in u3a_pile_done() the original, un-aligned cap.

[joemfb]

We need u3R->cap_p += pil_u->mov_ws to be aligned per lin_w (converted from bytes to words). Something like this in pile_prep (untested):

c3_dessert( lin_w & (lin_w - 1) ); 
c3_w lig_w = lin_w >> 2;
c3_w pad_w;

pil_u->bas_p = u3R->cap_p;

c3_w pad_w = (u3R->cap_p + pil_u->off_ws) & (lig_w - 1);

u3R->cap_p += wor_w - pad_w; // north?
u3R->cap_p -= pad_w; // south?

pil_u->top_p = u3R->cap_p;