Fix non-glibc build of the test suite with s390x_features

Fix non-glibc build of the test suite with s390x_features

s390x_features is built unconditionally on a range of platforms, accordingly
any non-portable or glibc-specific functionality must be guarded.

Fixes error reported when running 'make check' or 'make regtest' on a platform
with an alternative libc that Valgrind supports, in this case Apple's libc:

  s390x_features.c:13:10: fatal error: 'features.h' file not found
  #include <features.h>  // __GLIBC_PREREQ
           ^
  1 error generated.

Fixes: 161d22f0a ("s390x: Fix vector facility (vx) check in test suite")