S390: Define SINGLE_THREAD_BY_GLOBAL only on s390x

Starting with commit e070501d
"Replace __libc_multiple_threads with __libc_single_threaded"
the testcases nptl/tst-cancel-self and
nptl/tst-cancel-self-cancelstate are failing.

This is fixed by only defining SINGLE_THREAD_BY_GLOBAL on s390x,
but not on s390.

Starting with commit 09c76a74
"Linux: Consolidate {RTLD_}SINGLE_THREAD_P definition",
SINGLE_THREAD_BY_GLOBAL was defined in
sysdeps/unix/sysv/linux/s390/s390-64/sysdep.h.

Lateron the commit 9a973da6
"s390: Consolidate Linux syscall definition" consolidates the sysdep.h files
from s390-32/s390-64 subdirectories.  Unfortunately the macro is now always
defined instead of only on s390-64.

As information:
TLS_MULTIPLE_THREADS_IN_TCB is also only defined for s390.
See: sysdeps/s390/nptl/tls.h