Move append_insns out of aarch64_relocate_instruction

Move append_insns out of aarch64_relocate_instruction

aarch64_relocate_instruction should only decode instructions, and other
operations should be done out side of it.  This patch moves append_insns
out of aarch64_relocate_instruction, to its caller.

gdb/gdbserver:

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

	* linux-aarch64-low.c (aarch64_relocate_instruction): Return
	int.  Add argument buf.
	(aarch64_install_fast_tracepoint_jump_pad): Pass buf to
	aarch64_relocate_instruction.