S390: Add hwcaps value for vector facility.

The HWCAP_S390_VX flag in hwcap field of auxiliary vector indicates
if the vector facility is available and the kernel is aware of it.
This can be tested with LD_SHOW_AUXV=1 <prog>.
Currently it does not show te, because it was not incremented
by commit "S/390: Add hwcap value for transactional execution.".
Thus _DL_HWCAP_COUNT is incremented by two.

ChangeLog:

	* sysdeps/s390/dl-procinfo.c (_dl_s390_platforms): Add vector flag.
	* sysdeps/s390/dl-procinfo.h: Add vector capability.
	* sysdeps/unix/sysv/linux/s390/bits/hwcap.h (HWCAP_S390_VX): Define.

(cherry picked from commit 4e28fa80)