Fix incorrect roots order after root removal in remove_roots_subregion

Fix incorrect roots order after root removal in remove_roots_subregion
(fix of commits 38d194a, e849b45)

Issue #218 (bdwgc).

* mark_rts.c [USE_PROC_FOR_LIBRARIES] (swap_static_roots): New inline
function.
* mark_rts.c [USE_PROC_FOR_LIBRARIES] (GC_remove_roots_subregion):
Use swap_static_roots (after GC_add_roots_inner); if there is
a temporary root after GC_remove_root_at_pos then swap it with the
appropriate non-temporary one to ensure all temporary roots are
grouped at the end.