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:
    
    2017-06-07  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.