Fix alignment on StencilEntry struct for HIP

CUDA neatly aligns the StencilEntry struct to 16-bytes, which is not used on HIP.
Mapping the StencilEntry struct to 16-bytes on HIP yields around 5% better performance
on MI355X-class GPUs.

Grid/stencil/Stencil.h

@@ -82,7 +82,7 @@ void DslashLogPartial(void);
 void DslashLogDirichlet(void);
 
 struct StencilEntry {
-#ifdef GRID_CUDA
+#if defined(GRID_CUDA) || defined(GRID_HIP)
   uint64_t _byte_offset;       // 8 bytes
   uint32_t _offset;            // 4 bytes
 #else