Share i386-linux target description between GDB and GDBserver
    
    The code on creating i386-linux target descriptions are quite similar
    between GDB and GDBserver, so this patch moves them into a shared file
    arch/i386.c.  I didn't name it as i386-linux.c, because I want to reuse it
    to create other i386 non-linux target descriptions later.
    
    gdb:
    
    2017-07-05  Yao Qi  <yao.qi@linaro.org>
    
    	* Makefile.in (ALL_TARGET_OBS): Add i386.o.
    	(SFILES): Add arch/i386.c.
    	(HFILES_NO_SRCDIR): Add arch/i386.h.
    	* arch/i386.c: New file.
    	* arch/i386.h: New file.
    	* arch/tdesc.h (allocate_target_description): Declare.
    	(set_tdesc_architecture): Declare.
    	(set_tdesc_osabi): Declare.
    	* configure.tgt (i[34567]86-*-linux*): Add i386.o.
    	* i386-linux-tdep.c: Don't include ../features/i386/32bit-XXX.c.
    	include arch/i386.h.
    	(i386_linux_read_description): Remove code and call
    	i386_create_target_description.
    	(set_tdesc_architecture): New function.
    	(set_tdesc_osabi): New function.
    	* target-descriptions.h (allocate_target_description): Remove.
    
    gdb/gdbserver:
    
    2017-07-05  Yao Qi  <yao.qi@linaro.org>
    
    	* Makefile.in (arch-i386.o): New rule.
    	* configure.srv (i[34567]86-*-linux*): Add arch-i386.o.
    	(x86_64-*-linux*): Likewise.
    	* linux-x86-tdesc.c: Don't include ../features/i386/32bit-XXX.c,
    	include arch/i386.h.
    	(i386_linux_read_description): Remove code and call
    	i386_create_target_description.
    	* tdesc.c (allocate_target_description): New function.
    	* tdesc.h (set_tdesc_architecture): Remove declaration.
    	(set_tdesc_osabi): Likewise.