Replace dcache with splay tree.

	Replace dcache with splay tree.
	Remove partially implemented writeback support.
	* dcache.c: Include splay-tree.h.
	(LINE_SIZE_POWER): Change from 5 to 6.
	(DCACHE_SIZE): Change from 64 to 4096.
	(ENTRY_INVALID, ENTRY_VALID, ENTRY_DIRTY): Delete.
	(state_chars): Delete.
	(struct dcache_block): Clean up; remove state and anydirty fields.
	(struct dcache_struct): Redefine as a splay tree and linked list.
	(last_cache): Make static.
	(dcache_invalidate, dcache_hit): Rewrite for new cache structure.
	(dcache_read_line, dcache_alloc): Rewrite for new cache structure.
	(dcache_write_line): Delete.
	(dcache_writeback): Delete.
	(dcache_peek_byte): Clean up; remove "invalid" state check.
	(dcache_poke_byte): Rewrite for new cache structure; clarify comment.
	(dcache_splay_tree_compare): New function.
	(dcache_init, dcache_free): Rewrite for new cache structure.
	(dcache_xfer_memory): Rewrite for new write-through cache structure.
	(dcache_print_line): New function.
	(dcache_info): Rewrite for new cache structure.
	(_initialize_dcache): Update "info dcache" help text.
	* dcache.h (dcache_xfer_memory): Update declaration.
	* target.c (memory_xfer_partial): Update calls to dcache_xfer_memory.