Fix capability provenance in GC_prev_block on CHERI

(fix of commit c29365b)

Issue #627 (bdwgc).

Use a capability (pointer) rather than a computed pointer value
in `GC_prev_block()`.

* allchblk.c (GC_split_block): Initialize `last_hdr->hb_block`.
* alloc.c (GC_add_to_heap): Initialize `hhdr->hb_block`.
* headers.c (GC_prev_block): Remove TODO item; return `hhdr->hb_block`
instead of `HBLK_ADDR(bi,j)`; add assertion that `HBLK_ADDR(bi,j)`
gives the same address as of `hhdr->hb_block`.

allchblk.c

@@ -688,6 +688,7 @@ GC_split_block(struct hblk *hbp, hdr *hhdr, struct hblk *last_hbp,
   /* Replace hbp with last_hbp on its free list.  */
   last_hdr->hb_prev = prev;
   last_hdr->hb_next = next;
+  last_hdr->hb_block = last_hbp;
   last_hdr->hb_sz = hhdr->hb_sz - h_size;
   last_hdr->hb_flags = 0;
   if (prev /* != NULL */) { /* CPPCHECK */

alloc.c

@@ -1576,6 +1576,7 @@ GC_add_to_heap(struct hblk *h, size_t sz)
   GC_heap_sects[GC_n_heap_sects].hs_start = (ptr_t)h;
   GC_heap_sects[GC_n_heap_sects].hs_bytes = sz;
   GC_n_heap_sects++;
+  hhdr->hb_block = h;
   hhdr->hb_sz = sz;
   hhdr->hb_flags = 0;
   GC_freehblk(h);

headers.c

@@ -429,8 +429,8 @@ GC_prev_block(struct hblk *h)
       } else if (IS_FORWARDING_ADDR_OR_NIL(hhdr)) {
         j -= (GC_signed_word)ADDR(hhdr);
       } else {
-        /* TODO: return hhdr -> hb_block instead */
-        return (struct hblk *)MAKE_CPTR(HBLK_ADDR(bi, j));
+        GC_ASSERT(HBLK_ADDR(bi, j) == ADDR(hhdr->hb_block));
+        return hhdr->hb_block;
       }
     }
     j = BOTTOM_SZ - 1;