Fix GCC false warning

Fix GCC false warning

At least on
	gcc-4.4.7-11.el6.i686
	./configure --enable-64-bit-bfd --enable-targets=all
GDB does not build due to:
	cc1: warnings being treated as errors
	s390-linux-tdep.c: In function ‘s390_handle_arg’:
	s390-linux-tdep.c:2575: error: ‘val’ may be used uninitialized in this function

gdb/ChangeLog
2015-07-02  Jan Kratochvil  <jan.kratochvil@redhat.com>

	Fix GCC false warning.
	* s390-linux-tdep.c (s390_handle_arg): Initialize VAL.