Don't manually mess with sigaltstack on Android any more.

Bionic now takes care of things.

Bug: 8557703
Change-Id: I0510a193f2433eaf54c9c6ac3415c85b50a7f1d2
diff --git a/runtime/thread_android.cc b/runtime/thread_android.cc
index 7c4551f..73a9e54 100644
--- a/runtime/thread_android.cc
+++ b/runtime/thread_android.cc
@@ -87,4 +87,12 @@
   return managed_priority;
 }
 
+void Thread::SetUpAlternateSignalStack() {
+  // Bionic does this for us.
+}
+
+void Thread::TearDownAlternateSignalStack() {
+  // Bionic does this for us.
+}
+
 }  // namespace art