Make use of sigchainlib for signal chaining for implicit checks

This adds a preload library that overrides sigaction and sigprocmask
to implement signal chaining.  Signal chaining allows us to chain
any signal so that the ART runtime receives it before any signal
handler registered in native code by an application.  If the
ART signal handler doesn't want it, it will pass it on to the
user's handler.

ART uses signals for null pointer checks, stack overflow checks and
suspend points.

Also adds an OAT test to test this in isolation.

Change-Id: I9545f9f7343774c091410eb810504d9855fd399f
diff --git a/runtime/thread.h b/runtime/thread.h
index 9a7cb48..08bbcae 100644
--- a/runtime/thread.h
+++ b/runtime/thread.h
@@ -121,7 +121,7 @@
   // of the stack (lowest memory).  The higher portion of the memory
   // is protected against reads and the lower is available for use while
   // throwing the StackOverflow exception.
-  static constexpr size_t kStackOverflowProtectedSize = 32 * KB;
+  static constexpr size_t kStackOverflowProtectedSize = 16 * KB;
   static constexpr size_t kStackOverflowImplicitCheckSize = kStackOverflowProtectedSize +
     kStackOverflowReservedBytes;