[sanitizer] Force TLS allocation on s390

When running with an old glibc, CollectStaticTlsBlocks() calls
__tls_get_addr() in order to force TLS allocation. This function is not
available on s390 and the code simply does nothing in this case,
so all the resulting static TLS blocks end up being incorrect.

Fix by calling __tls_get_offset() on s390.

Reviewed By: dvyukov

Differential Revision: https://reviews.llvm.org/D105629