Mark new TSan test as unsupported on PPC.

Notably fails under PPC. For now, just exclude it. More details in the
original Phabricator review, https://reviews.llvm.org/D110552.

compiler-rt/test/tsan/lock_free_stack.cpp

@@ -1,5 +1,11 @@
 // RUN: %clangxx_tsan -O1 %s -o %t && %run %t 2>&1 | FileCheck %s
 // RUN: %clangxx_tsan -O1 %s -DRACE -o %t && %deflake %run %t | FileCheck %s --check-prefix=CHECK-RACE
+
+// Found in the post-submit testing under PPC (documented in
+// https://reviews.llvm.org/D110552), this test fails under PowerPC. Should be
+// investigated at some point.
+// UNSUPPORTED: ppc
+
 #include "test.h"
 
 const int kThreadCount = 4;