Test unwinding through a signal handler.

Also make backtrace_testlib.c C++.

Bug: 34468756
Test: Ran new unit test on host x86, host x86_64, angler (32 bit, 64 bit),
Test: fugu.
Change-Id: Ia810f596c2df56cd56e8ab17e6c19be0c48d737f
diff --git a/libbacktrace/Android.bp b/libbacktrace/Android.bp
index 0e7c6f3..8f74a1a 100644
--- a/libbacktrace/Android.bp
+++ b/libbacktrace/Android.bp
@@ -114,7 +114,7 @@
         none: true,
     },
     cflags: ["-O0"],
-    srcs: ["backtrace_testlib.c"],
+    srcs: ["backtrace_testlib.cpp"],
 
     target: {
         linux: {