Fix infinite restarting of mark_some when a static root disappeared (Linux)

Fix infinite restarting of mark_some when a static root disappeared (Linux)
(a cherry-pick of commits 38d194a3, e849b45c from 'master')

Issue #218 (bdwgc).

* dyn_load.c [USE_PROC_FOR_LIBRARIES] (GC_register_map_entries): Call
GC_remove_roots_subregion for each region which has prot starting
with "---"; add comment.
* include/private/gc_priv.h [USE_PROC_FOR_LIBRARIES]
(GC_remove_roots_subregion): Declare function.
* mark_rts.c [USE_PROC_FOR_LIBRARIES] (GC_remove_roots_subregion):
Implement (including partial overlapping).