Remove last_cache

Remove last_cache

This patch removes global variable 'last_cache', and initialize
'target_dcache' lazily, so that 'target_dcache' can replace
'last_cache'.  No functionalities should be changed after this patch.

gdb:

2013-11-20  Yao Qi  <yao@codesourcery.com>

	* dcache.c (last_cache): Remove.
	(dcache_free, dcache_init): Update.
	(dcache_update):
	(dcache_print_line): Add parameter 'dcache'.  Replace
	'target_dcache' with 'dcache'.
	(dcache_info): Move code to dcache_info_1. Call
	'dcache_info_1'.
	(dcache_info_1): New function.
	(set_dcache_size): Call target_dcache_invalidate.
	(set_dcache_line_size): Call target_dcache_invalidate.
	* target.c (target_dcache_init_p): New function.
	(target_dcache_invalidate): Check target_dcache_init_p first.
	(target_dcache_get, target_dcache_get_or_init): New function.
	(memory_xfer_partial_1): Adjust.
	(initialize_target): Don't initialize 'target_dcache'.
	* target.h (struct dcache_struct): Declare.
	(target_dcache_get): Declare.