Use arm target description and regs_info for 32-bit file on aarch64 GDBserver

Use arm target description and regs_info for 32-bit file on aarch64 GDBserver

This patch teaches aarch64-linux GDBserver use 32-bit arm target
description and regs_info if the elf file is 32-bit.

gdb/gdbserver:

2015-08-04  Yao Qi  <yao.qi@linaro.org>

	* configure.srv (case aarch64*-*-linux*): Append arm-with-neon.o
	to srv_regobj and append arm-core.xml arm-vfpv3.xml and
	arm-with-neon.xml to srv_xmlfiles.
	* linux-aarch64-low.c: Include linux-aarch32-low.h.
	(is_64bit_tdesc): New function.
	(aarch64_linux_read_description): New function.
	(aarch64_arch_setup): Call aarch64_linux_read_description.
	(regs_info): Rename to regs_info_aarch64.
	(aarch64_regs_info): Return right regs_info.
	(initialize_low_arch): Call initialize_low_arch_aarch32.