S390: Use cu42 instruction for converting from utf32 to utf16.

This patch adds an ifunc variant to use the cu instruction on arch12 CPUs.
This new ifunc variant can be built if binutils support z13 vector
instructions.  At runtime, HWCAP_S390_VXE decides if we can use the
cu42 instruction.

ChangeLog:

	* sysdeps/s390/utf16-utf32-z9.c (__to_utf16_loop_vx_cu):
	Use vector and cu42 instruction.
	* sysdeps/s390/multiarch/utf16-utf32-z9.c:
	Add __to_utf16_loop_vx_cu in ifunc resolver.