ubsan: disable sigaction test for aarch64

The test seems to segfault on aarch64 with tsan:
https://lab.llvm.org/buildbot/#/builders/179/builds/6662
Reason unknown, needs debugging.

Reviewed By: melver, Caslyn

Differential Revision: https://reviews.llvm.org/D155272

Author: dvyukov

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

@@ -2,6 +2,11 @@
 
 // Ensure ubsan runtime/interceptors are lazily initialized if called early.
 
+// 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{{.*}}
+
 #include <assert.h>
 #include <signal.h>
 #include <stdio.h>