S390: Use cu24 instruction for converting from utf16 to utf32.

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
cu24 instruction.

ChangeLog:

	* sysdeps/s390/utf16-utf32-z9.c (__from_utf16_loop_vx_cu):
	Use vector and cu24 instruction.
    	* sysdeps/s390/multiarch/utf16-utf32-z9.c:
    	Add __from_utf16_loop_vx_cu in ifunc resolver.