u3: fix alignment in u3a_pile_prep

[yosoyubik]

[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;