libbacktrace: remove exit time destructors.

If libbacktrace is statically linked into a library that interposes
libc, exit-time destructors can lead to a segfault upon exit if static
variables are used after their destruction.

Remove the one use of a static variable (with inconsequential
performance impact of less than 1us per backtrace), and enable
-Wexit-time-destructors to ensure this stays fixed in the future.

Test: mma
Change-Id: Icdaf1cf1c1f166cf501d9ecd1983c44e4305f7e9

libbacktrace/Android.bp

@@ -73,6 +73,10 @@ cc_library {
     defaults: ["libbacktrace_common"],
     host_supported: true,
 
+    cflags: [
+        "-Wexit-time-destructors",
+    ],
+
     srcs: [
         "BacktraceMap.cpp",
     ],

libbacktrace/UnwindStack.cpp

@@ -45,7 +45,7 @@
 
 bool Backtrace::Unwind(unwindstack::Regs* regs, BacktraceMap* back_map,
                        std::vector<backtrace_frame_data_t>* frames, size_t num_ignore_frames) {
-  static std::vector<std::string> skip_names{"libunwindstack.so", "libbacktrace.so"};
+  std::vector<std::string> skip_names{"libunwindstack.so", "libbacktrace.so"};
   UnwindStackMap* stack_map = reinterpret_cast<UnwindStackMap*>(back_map);
   auto process_memory = stack_map->process_memory();
   unwindstack::Unwinder unwinder(MAX_BACKTRACE_FRAMES + num_ignore_frames, stack_map->stack_maps(),

libunwindstack/Android.bp

@@ -65,6 +65,10 @@ cc_library {
         "Symbols.cpp",
     ],
 
+    cflags: [
+        "-Wexit-time-destructors",
+    ],
+
     target: {
         // Always disable optimizations for host to make it easier to debug.
         host: {