S390: Add vector register support to gdbserver

S390: Add vector register support to gdbserver

On S/390 targets with vector registers, enable gdbserver to advertise
and handle the feature "org.gnu.gdb.s390.vx".

gdb/gdbserver/ChangeLog:

	* Makefile.in (s390-vx-linux64.c, s390-tevx-linux64.c)
	(s390x-vx-linux64.c, s390x-tevx-linux64.c): New rules.
	(clean): Add "rm -f" for above C files.
	* configure.srv (srv_regobj): Add s390-vx-linux64.o,
	s390-tevx-linux64.o, s390x-vx-linux64.o, and s390x-tevx-linux64.o.
	(srv_xmlfiles): Add s390-vx-linux64.xml, s390-tevx-linux64.xml,
	s390x-vx-linux64.xml, s390x-tevx-linux64.xml, and s390-vx.xml.
	* linux-s390-low.c (HWCAP_S390_VX): New macro.
	(init_registers_s390_vx_linux64, init_registers_s390_tevx_linux64)
	(init_registers_s390x_vx_linux64)
	(init_registers_s390x_tevx_linux64)
	(tdesc_s390_vx_linux64, tdesc_s390_tevx_linux64)
	(tdesc_s390x_vx_linux64, tdesc_s390x_tevx_linux64): New extern
	declarations.
	(s390_fill_vxrs_low, s390_store_vxrs_low, s390_fill_vxrs_high)
	(s390_store_vxrs_high): New functions.
	(s390_regsets): Add entries for NT_S390_VXRS_LOW and
	NT_S390_VXRS_HIGH.
	(s390_arch_setup): Add logic for selecting one of the new target
	descriptions.  Activate the new vector regsets if applicable.
	(initialize_low_arch): Also invoke init_registers_s390_vx_linux64,
	init_registers_s390_tevx_linux64, init_registers_s390x_vx_linux64,
	and init_registers_s390x_tevx_linux64.