Move target_read_uint32 out of aarch64_relocate_instruction

Move target_read_uint32 out of aarch64_relocate_instruction

This patch is to move target_read_uint32 out of
aarch64_relocate_instruction and pass INSN to
aarch64_relocate_instruction, so that it is cleaner, only decode
instructions.

gdb/gdbserver:

2015-10-12  Yao Qi  <yao.qi@linaro.org>

	* linux-aarch64-low.c (aarch64_relocate_instruction): Add
	argument insn.  Remove local variable insn.  Don't call
	target_read_uint32.
	(aarch64_install_fast_tracepoint_jump_pad): Call
	target_read_uint32.