[aarch64] use aarch64_decode_insn to decode instructions in GDB

[aarch64] use aarch64_decode_insn to decode instructions in GDB

In this patch, we start to use aarch64_decode_insn to decode instructions
in aarch64_software_single_step.

gdb:

2015-10-07  Yao Qi  <yao.qi@linaro.org>

	* aarch64-tdep.c: Include opcode/aarch64.h.
	(submask): Move it above.
	(bit): Likewise.
	(bits): Likewise.
	(aarch64_software_single_step): Call aarch64_decode_insn.
	Decode instruction by aarch64_inst instead of using
	aarch64_decode_bcond and decode_masked_match.