[powerpc] deactivate ManyThreadsTest asan test on powerpc64
This test case occassionally hangs when run on powerpc. This is also a
problem on AArch64 (see https://bugs.llvm.org/show_bug.cgi?id=24389).
Reactivate this when the problem is fixed.
This could also be related to the same problem as with the tests
ThreadedOneSizeMallocStressTest, ThreadedMallocStressTest, and several
others that do not run reliably on powerpc.