Release memory-block-pool memory back to malloc.

Release memory-block-pool memory back to malloc.

	* ggc-page.c (ggc_collect): Call memory_block_pool::trim.
	* memory-block.cc (memory_block_pool::clear_free_list): Rename to ...
	(memory_block_pool::reduce_free_list): ... this one.
	(memory_block_pool::trim): New static function.
	* memory-block.h (memory_block_pool::freelist_size): New constant
	(memory_block_pool::clear_free_list): Rename to ...
	(memory_block_pool::reduce_free_list): ... this one.
	(memory_block_pool::trim): Declare.

	* lto.c (lto_wpa_write_files): Call memory_block_pool::trim.

From-SVN: r278616