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.