Move dwarf2_frame_state_reg.exp_len to union .loc

Move dwarf2_frame_state_reg.exp_len to union .loc

dwarf2_frame_state_reg.exp_len is only used together with .loc.exp, so
it makes more sense to exp_len to the union as well.

gdb:

2017-08-11  Yao Qi  <yao.qi@linaro.org>

	* dwarf2-frame.h (struct dwarf2_frame_state_reg) <exp_len>: Remove.
	<loc.exp>: New field.
	* dwarf2-frame.c (execute_cfa_program): Update.
	(dwarf2_frame_prev_register): Update.