UserspaceEmulator: Capture backtraces of malloc/free events

This lets us show backtraces for each leaked mallocation in the leak
report at the end. :^)

DevTools/UserspaceEmulator/Emulator.h

@@ -46,6 +46,8 @@ public:
 
     bool load_elf();
     void dump_backtrace();
+    void dump_backtrace(const Vector<FlatPtr>&);
+    Vector<FlatPtr> raw_backtrace();
 
     int exec();
     u32 virt_syscall(u32 function, u32 arg1, u32 arg2, u32 arg3);