Don't scan prologue past epilogue

This patch is to stop prologue analysis past epilogue in for arm mode,
while we've already had done the same to thumb mode (see
thumb_instruction_restores_sp).  This is useful to parse functions
with empty body (epilogue follows prologue).

gdb:

2014-12-12  Yao Qi  <yao@codesourcery.com>

	* arm-tdep.c (arm_instruction_restores_sp): New function.
	(arm_analyze_prologue): Call arm_instruction_restores_sp.
	(arm_in_function_epilogue_p): Move code to
	arm_instruction_restores_sp.