Use code cache in aarch64 prologue analyzer

Use code cache in aarch64 prologue analyzer

This patch change aarch 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 when
the program is compiled without debug information.

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

/			11.1635239124	9.99472999573	9.65339517593	9.66648793221
-fstack-protector-all	11.2560930252	9.338118	9.63896489143	9.59474396706

gdb:

2016-12-9  Yao Qi  <yao.qi@linaro.org>

	* aarch64-tdep.c (instruction_reader::read): Call
	read_code_unsigned_integer instead of
	read_memory_unsigned_integer.