Fix visibility of some GC internal symbols used by GCJ

Fix visibility of some GC internal symbols used by GCJ
(Includes reverting part of commits 68b9f27, 91d5df5, b2345fb)

* allchblk.c (GC_hblkfreelist, GC_free_bytes): Do not define array as
STATIC again but only if GC_GCJ_SUPPORT (for GNU GCJ client).
* blacklst.c (GC_is_black_listed): Do not define as GC_INNER again
(for GNU GCJ client).
* include/private/gc_priv.h (GC_is_black_listed): Likewise.
* include/private/gc_priv.h (GC_hblkfreelist): Restore removed
declaration (but only if GC_GCJ_SUPPORT).
* reclaim.c (GC_n_set_marks): Do not define as STATIC again (for GCJ);
update the comment.