set/show code-cache

set/show code-cache

Similar to stack cache, in this patch, we add
TARGET_OBJECT_CODE_MEMORY to read code from target and add a new
option "set code-cache on|off" to optimize code accesses by
using the target memory cache.

In V4:
 - Remove "without affecting correctness" from NEWS and doc.
 - Replace "ON" with "on" in doc.
 - "access" -> "accesses".

In V3:
 - Rename functions and variables.
 - Update command help, doc and NEWS entry.
 - Invalidate cache on option transitions, to align with
   the behaviour of "stack-cache".  Since cache invalidation is
   transparent to users, users don't know option "stack-cache"
   transitions cause code cache invalidation.

V2 was reviewed by Doug.  There are some changes in V3, so I post it
here.

gdb:

2013-11-24  Yao Qi  <yao@codesourcery.com>

	* NEWS: Add note on new "set code-cache" option.
	* target-dcache.c (code_cache_enabled_1): New variable.
	(code_cache_enabled): New variable.
	(show_code_cache, set_code_cache): New function.
	(code_cache_enabled_p): New function.
	(_initialize_target_dcache): Register command.
	* target-dcache.h (code_cache_enabled_p): Declare.
	* target.c (memory_xfer_partial_1):Handle
	TARGET_OBJECT_CODE_MEMORY and code_cache_enabled.
	(target_read_code): New function.
	* target.h (enum target_object) <TARGET_OBJECT_CODE_MEMORY>:
	New.
	(target_read_code): Declare.

gdb/doc:

2013-11-24  Yao Qi  <yao@codesourcery.com>

	* gdb.texinfo (Caching Remote Data): Document new
	"set/show stack-cache" option.