Add compat_str() which works with unicode in both Python 2 and 3

GDB can be built with Python 2 or with Python 3

src/etc/gdb_rust_pretty_printing.py

@@ -78,7 +78,7 @@ class GdbValue(rustpp.Value):
 
     def as_integer(self):
         if self.gdb_val.type.code == gdb.TYPE_CODE_PTR:
-            as_str = unicode(self.gdb_val).split()[0]
+            as_str = rustpp.compat_str(self.gdb_val).split()[0]
             return int(as_str, 0)
         return int(self.gdb_val)