selftests: vDSO: fix ELF hash table entry size for s390x

The vDSO self tests fail on s390x for a vDSO linked with the GNU linker
ld as follows:

  # ./vdso_test_gettimeofday
  Floating point exception (core dumped)

On s390x the ELF hash table entries are 64 bits instead of 32 bits in
size (see Glibc sysdeps/unix/sysv/linux/s390/bits/elfclass.h).

Fixes: 40723419 ("kselftest: Enable vDSO test on non x86 platforms")
Reported-by: Heiko Carstens <hca@linux.ibm.com>
Tested-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Jens Remus <jremus@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>