Remove unused variables from gdbserver

Remove unused variables from gdbserver

This removes a few unused variables from gdbserver.

The x86-tdesc.h change is a bit unusual for this series.  This file
was not defining the multiple-include guard symbol, so I've added that
here.  Also, it is hard to determine when i386_expedite_regs will be
needed, so this patch simply marks it ATTRIBUTE_UNUSED.

gdb/gdbserver/ChangeLog
2018-07-22  Tom Tromey  <tom@tromey.com>

	* win32-low.c (win32_create_inferior): Remove unused variables.
	* gdbreplay.c (remote_open): Remove unused variable.
	* remote-utils.c (remote_prepare): Remove unused variable.
	* x86-tdesc.h (X86_TDESC_H): Define.
	(amd64_expedite_regs): Define conditionally.
	(i386_expedite_regs): Mark ATTRIBUTE_UNUSED.
	* linux-x86-tdesc.c (i386_tdescs): Move inside #if.
	* remote-utils.c (readchar): Remove unused variable.