Move aarch64_relocate_instruction to arch/aarch64-insn.c

Move aarch64_relocate_instruction to arch/aarch64-insn.c

This patch moves aarch64_relocate_instruction and visitor class to
arch/aarch64-insn.c, so that both GDB and GDBserver can use it.

gdb:

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

	* arch/aarch64-insn.c (aarch64_decode_ldr_literal): Moved from
	gdbserver/linux-aarch64-low.c.
	(aarch64_relocate_instruction): Likewise.
	* arch/aarch64-insn.h (aarch64_decode_ldr_literal): Declare.
	(struct aarch64_insn_data): Moved from
	gdbserver/linux-aarch64-low.c.
	(struct aarch64_insn_visitor): Likewise.
	(aarch64_relocate_instruction): Declare.

gdb/gdbserver:

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

	* linux-aarch64-low.c (extract_signed_bitfield): Remove.
	(aarch64_decode_ldr_literal): Move to gdb/arch/aarch64-insn.c.
	(aarch64_relocate_instruction): Likewise.
	(struct aarch64_insn_data): Move to gdb/arch/aarch64-insn.h.
	(struct aarch64_insn_visitor): Likewise.