[aarch64] expose disas_aarch64_insn and rename it to aarch64_decode_insn

[aarch64] expose disas_aarch64_insn and rename it to aarch64_decode_insn

We want to use disas_aarch64_insn inside GDB to decode instructions, so
this patch exposes it and rename it to aarch64_decode_insn to follow
the conventions of other interfaces.

include/opcode:

2015-10-02  Yao Qi  <yao.qi@linaro.org>

	* aarch64.h (aarch64_decode_insn): Declare it.

opcodes:

2015-10-02  Yao Qi  <yao.qi@linaro.org>

	* aarch64-dis.c	(disas_aarch64_insn): Remove static.  Change
	argument insn type to aarch64_insn.  Rename to ...
	(aarch64_decode_insn): ... it.
	(print_insn_aarch64_word): Caller updated.