In Gitlab CI, some ppc64 multi-threaded tcg tests crash when run in the
clang-user job with an assertion failure in glibc that seems to
indicate corruption:
signals: allocatestack.c:223: allocate_stack:
Assertion `powerof2 (pagesize_m1 + 1)' failed.
Disable these tests for now.
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>