Centralize amd64-linux target descriptions

This patch adds a new function amd64_linux_read_description, which
creates amd64-linux target descriptions according to its two
arguments, xcr0 and is_x32.

gdb:

2017-09-05  Yao Qi  <yao.qi@linaro.org>

	* amd64-linux-tdep.c (amd64_linux_read_description): New
	function.
	(amd64_linux_core_read_description): Call
	amd64_linux_read_description.
	(amd64_linux_init_abi): Likewise.
	(amd64_x32_linux_init_abi): Likewise.
	* amd64-linux-tdep.h (amd64_linux_read_description): Declare.
	* x86-linux-nat.c (x86_linux_read_description): Call
	amd64_linux_read_description.