implement a replacement for if_marked

implement a replacement for if_marked

gcc/ChangeLog:

2014-11-20  Trevor Saunders  <tsaunders@mozilla.com>

	* doc/gty.texi: Document the new cache gty attribute.
	* gengtype.c (finish_cache_funcs): New function.
	(write_roots): Call gt_clear_cache on global variables with the cache
	gty attribute.
	* ggc-common.c (ggc_mark_roots): Call gt_clear_caches.
	* ggc.h (gt_clear_caches): New declaration.
	* hash-table.h (struct ggc_cache_hasher): New hasher for caches in gc
	memory.
	(gt_cleare_cache): New function.
	* emit-rtl.c, rtl.h, tree.c: Use hash_table instead of htab.

From-SVN: r217866