Add new register access interface to expr.c

Add new register access interface to expr.c

DWARF expression evaluator is currently using get_frame_register_bytes
and put_frame_register_bytes interface for register access.

The problem with evaluator using this interface is that it allows a
bleed out register access. This means that if the caller specifies a
larger amount of data then the size of a specified register, the
operation will continue accessing the neighboring registers until a
full amount of data has been reached.

DWARF specification does not define this behavior, so a new simplified
register access interface is needed instead.

	* dwarf2/expr.c (read_from_register): New function.
	(write_to_register): New function.
	(rw_pieced_value): Now calls the read_from_register and
	write_to_register functions.

Change-Id: I885b6a02353bc3fcf0dd5c600b5f1a73a7f9e340