Move aarch64-insn.o to arch/aarch64-insn.o and Remove a rule for arch/*.c

Move aarch64-insn.o to arch/aarch64-insn.o and Remove a rule for arch/*.c

This patch moves aarch64-insn.o to arch/aarch64-insn.o.  Then, all
arch/*.c are built to arch/*.o, so we don't need a Makefile rule to build
*.o from arch/*.c.  This patch removes it too.

gdb:

2017-10-06  Yao Qi  <yao.qi@linaro.org>

	* Makefile.in (ALL_64_TARGET_OBS): Replace aarch64-insn.o with
	arch/aarch64-insn.o.
	Remove one rule.
	* configure.tgt: Replace aarch64-insn.o with arch/aarch64-insn.o.