Use amd64_target_description to get tdesc_amd64
    
    This patch changes amd64-*-tdep.c files to use function
    amd64_target_description to get the right target description rather than
    use the variable tdesd_amd64.
    
    gdb:
    
    2017-08-18  Yao Qi  <yao.qi@linaro.org>
    
    	* amd64-darwin-tdep.c: Include "x86-xstate.h".
    	(x86_darwin_init_abi_64): Call amd64_target_description.
    	* amd64-dicos-tdep.c: Likewise.
    	* amd64-fbsd-nat.c: Likewise.
    	* amd64-fbsd-tdep.c: Likewise.
    	* amd64-nbsd-tdep.c: Likewise.
    	* amd64-obsd-tdep.c: Likewise.
    	* amd64-sol2-tdep.c: Likewise.
    	* amd64-windows-tdep.c: Likewise.
    	* amd64-tdep.h (tdesc_amd64): Remove the declaration.