Fix fallback signal issue.

Add signo for target thread in fallback path;
Update test for seccomp tombstone thread abort.

Bug: 336946834

Test: debuggerd_test
Test: Send fatal signal to process with NO_NEW_PRIVS
Change-Id: Ie9d77a93da9cd89ab7093b8949f311e03d96ec50
diff --git a/debuggerd/debuggerd_test.cpp b/debuggerd/debuggerd_test.cpp
index 7c52e6e..e4e2f99 100644
--- a/debuggerd/debuggerd_test.cpp
+++ b/debuggerd/debuggerd_test.cpp
@@ -1667,6 +1667,9 @@
 
   std::string result;
   ConsumeFd(std::move(output_fd), &result);
+  ASSERT_MATCH(
+      result,
+      R"(signal 6 \(SIGABRT\))");
   ASSERT_BACKTRACE_FRAME(result, "abort");
 }