Fix 'GC_push_marked1 discards const qualifier' gcc error

(fix of commit 1ab17de)

Issue #629 (bdwgc)

* mark.c [USE_PUSH_MARKED_ACCELERATORS] (GC_push_marked1): Specify hhdr
argument as const pointer.
* mark.c [USE_PUSH_MARKED_ACCELERATORS && !UNALIGNED_PTRS]
(GC_push_marked2): Likewise.
* mark.c [USE_PUSH_MARKED_ACCELERATORS && !UNALIGNED_PTRS
&& GC_GRANULE_WORDS<4] (GC_push_marked4): Likewise.

mark.c

@@ -1741,7 +1741,7 @@ GC_INNER void GC_push_all_stack(ptr_t bottom, ptr_t top)
 /* Push all objects reachable from marked objects in the given block */
 /* containing objects of size 1 granule.                             */
 GC_ATTR_NO_SANITIZE_THREAD
-STATIC void GC_push_marked1(struct hblk *h, hdr *hhdr)
+STATIC void GC_push_marked1(struct hblk *h, const hdr *hhdr)
 {
     const word *mark_word_addr = &(hhdr -> hb_marks[0]);
     word *p;
@@ -1795,7 +1795,7 @@ STATIC void GC_push_marked1(struct hblk *h, hdr *hhdr)
 /* Push all objects reachable from marked objects in the given block */
 /* of size 2 (granules) objects.                                     */
 GC_ATTR_NO_SANITIZE_THREAD
-STATIC void GC_push_marked2(struct hblk *h, hdr *hhdr)
+STATIC void GC_push_marked2(struct hblk *h, const hdr *hhdr)
 {
     const word *mark_word_addr = &(hhdr -> hb_marks[0]);
     word *p;
@@ -1847,7 +1847,7 @@ STATIC void GC_push_marked2(struct hblk *h, hdr *hhdr)
 /* There is a risk of mark stack overflow here.  But we handle that. */
 /* And only unmarked objects get pushed, so it's not very likely.    */
 GC_ATTR_NO_SANITIZE_THREAD
-STATIC void GC_push_marked4(struct hblk *h, hdr *hhdr)
+STATIC void GC_push_marked4(struct hblk *h, const hdr *hhdr)
 {
     const word *mark_word_addr = &(hhdr -> hb_marks[0]);
     word *p;