Use code cache in arm prologue analyzer

Use code cache in arm prologue analyzer

This patch change arm prologue analyzer using code cache, in order
to improve the performance of remote debugging.

gdb.perf/skip-prologue.exp (measured by wall-time) is improved a lot,

			Original	Patched		Original	Patched
			without dbg	without dbg	with dbg	with dbg

-marm			14.166741848	9.32852292061  	11.4908499718  	9.16302204132
-marm   		14.6705040932  	9.34849786758  	18.2788009644  	9.14823913574
\-fstack-protector-all
-mthumb			34.4391930103	10.6062178612 	13.7886838913	10.3094120026
-mthumb
\-fstack-protector-all	34.9310460091	10.6413481236	25.3875930309	10.6294929981

gdb:

2016-12-09  Yao Qi  <yao.qi@linaro.org>

	* arm-tdep.c (skip_prologue_function): Call
	read_code_unsigned_integer instead of
	read_memory_unsigned_integer.
	(thumb_analyze_prologue): Likewise.
	(arm_analyze_load_stack_chk_guard): Likewise.
	(arm_skip_stack_protector): Likewise.
	(arm_analyze_prologue):Likewise.
	(extend_buffer_earlier): Call target_read_code instead
	of target_read_memory.
	(arm_adjust_breakpoint_address): Likewise.