Running vdso_test_correctness on s390x (aka s390 64 bit) emits a warning:

Warning: failed to find clock_gettime64 in vDSO

This is caused by the "#elif defined (__s390__)" check in vdso_config.h
which the defines VDSO_32BIT.

If __s390x__ is defined also __s390__ is defined. Therefore the correct
check must make sure that only __s390__ is defined.

Therefore add the missing !defined(__s390x__). Also use common
__s390x__ define instead of __s390X__.
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Fixes: 693f5ca0 ("kselftest: Extend vDSO selftest")
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>