nptl: xfail tst-cleanup2 when building with ubsan

nptl: xfail tst-cleanup2 when building with ubsan

The test explicit uses UB (sprintf (NULL, ...) to triggers a SIGSEGV
from libc and check with the pthread cleanup are correctly called.

With ubsan enable, it triggers:

UBSAN: Undefined behaviour in Xprintf_buffer_write.c:39:7 null pointer passed as argument 1, nonnull attribute declared at unknown:0:0

Which is a memcpy call with NULL argument.