Add new target_read_raw_memory function, and consolidate comments.

Add new target_read_raw_memory function, and consolidate comments.

Tested on x86_64 Fedora 17.

gdb/
2013-12-02  Pedro Alves  <palves@redhat.com>

	* dcache.c (dcache_read_line): Use target_read_raw_memory.
	* target.c (target_read_raw_memory): New function.
	(target_read_stack, target_write_memory, target_write_raw_memory):
	Update comment.
	(target_read_code): Add comment.
	* target.h (target_read_raw_memory): Declare.