Broaden scope of gAborting. Fixes hang in HandleUnexpectedSignal.

Change-Id: I4c29cd7b67f07bb9f99c308feac5a3d6c236bc2b
diff --git a/src/runtime_linux.cc b/src/runtime_linux.cc
index 01c08d3..430c70f 100644
--- a/src/runtime_linux.cc
+++ b/src/runtime_linux.cc
@@ -228,6 +228,7 @@
 };
 
 static void HandleUnexpectedSignal(int signal_number, siginfo_t* info, void* raw_context) {
+  gAborting = true;  // set before taking any locks
   MutexLock mu(Thread::Current(), *Locks::unexpected_signal_lock_);
 
   bool has_address = (signal_number == SIGILL || signal_number == SIGBUS ||