Remove x32 non-linux target descriptions

x32 non-linux target descriptions are not used in GDB or GDBserver.  This
patch removes them.

gdb:

2017-08-09  Yao Qi  <yao.qi@linaro.org>

	* amd64-tdep.h (tdesc_x32): Remove the declaration.
	* amd64-tdep.c: Don't include features/i386/x32*.c.
	(_initialize_amd64_tdep): Don't call initialize_tdesc_x32*
	functions.
	* features/Makefile (WHICH): Remove i386/x32, i386/x32-avx,
	and i386/x32-avx-avx512.
	(XMLTOC): Remove i386/x32-avx.xml, i386/x32-avx-avx512.xml,
	and i386/x32.xml.
	* features/i386/x32-avx-avx512.c: Removed.
	* features/i386/x32-avx-avx512.xml: Removed.
	* features/i386/x32-avx.c: Removed.
	* features/i386/x32-avx.xml: Removed.
	* features/i386/x32.c: Removed.
	* features/i386/x32.xml: Removed.
	* regformats/i386/x32-avx-avx512.dat: Removed.
	* regformats/i386/x32-avx.dat: Removed.
	* regformats/i386/x32.dat: Removed.

gdb/gdbserver:

2017-08-09  Yao Qi  <yao.qi@linaro.org>

	* configure.srv (srv_amd64_regobj): Remove x32.o, x32-avx.o
	and x32-avx-avx512.o.
	(srv_amd64_xmlfiles): Remove i386/x32.xml, i386/x32-avx.xml
	i386/x32-avx-avx512.xml.