Rename regcache_cooked_read_ftype and make a function_view

regcache_cooked_read_ftype can be converted to a function_view, which
allows us to use lambda functions and therefore avoid having to pass an
opaque pointer parameter.

Adjusting the fallouts showed that the "const regcache &" passed to the
readonly_detached_regcache constructor is cast to non-const in
do_cooked_read.  I changed the constructor parameter to be non-const.

Finally, I renamed the typedef from regcache_cooked_read_ftype to
register_read_ftype, since there is nothing that forces us to use it
only for regcaches nor cooked registers.

gdb/ChangeLog:

	* regcache.h (regcache_cooked_read_ftype): Rename to...
	(register_read_ftype): ...this, change type to function_view.
	(class reg_buffer) <save>: Remove src parameter.
	(readonly_detached_regcache) <readonly_detached_regcache>: Make
	parameter non-const in first overload.  Remove src parameter in
	second overload.
	* regcache.c (do_cooked_read): Remove.
	(readonly_detached_regcache::readonly_detached_regcache): Make
	parameter non-const, adjust call to other constructor.
	(reg_buffer::save): Remove src parameter.
	* frame.c (do_frame_register_read): Remove.
	(frame_save_as_regcache): Use lambda function.
	* ppc-linux-tdep.c (ppu2spu_unwind_register): Change type of src
	parameter to ppu2spu_data *.
	(ppu2spu_sniffer): Use lambda function.