IBM zSystems: Hardcode HWCAP_S390_VXRS

IBM zSystems: Hardcode HWCAP_S390_VXRS

Compiling zlib-ng with glibc 2.17 (minimum version still supported by
crosstool-ng) fails due to the lack of HWCAP_S390_VX - it was
introduced in glibc 2.23.

Strictly speaking, this is a problem with the feature detection logic
in cmake. However, it's not worth disabling the s390x vectorized CRC32
if the hwcap constant is missing and the compiler intrinsics are
available.

So fix by hardcoding the constant. It's a part of the kernel ABI,
which does not change.