[ubsan] Make sigaction.cpp test UNSUPPORTED rather than XFAIL

The test fails on some builders but not on others; there's likely some
kind of environment dependence that should be investigated.

See https://reviews.llvm.org/D155317

compiler-rt/test/ubsan/TestCases/Misc/Linux/sigaction.cpp

@@ -5,7 +5,7 @@
 // The test seems to segfault on aarch64 with tsan:
 // https://lab.llvm.org/buildbot/#/builders/179/builds/6662
 // Reason unknown, needs debugging.
-// XFAIL: target=aarch64{{.*}} && ubsan-tsan
+// UNSUPPORTED: target=aarch64{{.*}} && ubsan-tsan
 
 #include <assert.h>
 #include <signal.h>