Centralize i386 linux target descriptions

This patch moves all the tdesc_i386*_linux target descriptions to a
function i386_linux_read_description, which returns the right target
description according to xcr0.  This also remove the duplication in
getting target descriptions in corefile and native target.

gdb:

2017-07-26  Yao Qi  <yao.qi@linaro.org>

	* i386-linux-tdep.c (i386_linux_read_description): New function.
	(i386_linux_core_read_description): Call
	i386_linux_read_description.
	* i386-linux-tdep.h (i386_linux_read_description): Declare.
	(tdesc_i386_linux, tdesc_i386_mmx_linux): Remove declarations.
	(tdesc_i386_avx_linux, tdesc_i386_mpx_linux): Likewise
	(tdesc_i386_avx_mpx_linux, tdesc_i386_avx_avx512_linux): Likewise.
	(tdesc_i386_avx_mpx_avx512_pku_linux): Likewise.
	* x86-linux-nat.c (x86_linux_read_description): Call
	i386_linux_read_description.