Generate s390 target description c files

Generate s390 target description c files

If I delete all target description c files under features/ directory,
and run make GDB=/scratch/yao/gdb/build-git/all-targets/gdb/gdb cfiles,
some s390 target description c files are not generated.

This patch adds these s390 xml files to XMLTOC, so these c files can
be generated.

gdb:

2016-10-05  Yao Qi  <yao.qi@linaro.org>

	* features/Makefile (XMLTOC): Add s390-tevx-linux64.xml,
	s390-vx-linux64.xml, s390x-tevx-linux64.xml and
	s390x-vx-linux64.xml.