s390x GCC and SystemZ Clang have ThreadSanitizer support now [1] [2],
so enable KCSAN for s390.

[1] https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=ea22954e7c58
[2] https://reviews.llvm.org/D105629Signed-off-by: NIlya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: NHeiko Carstens <hca@linux.ibm.com>