gdb: add inferior::{arch, set_arch}

Make the inferior's gdbarch field private, and add getters and setters.
This helped me by allowing putting breakpoints on set_arch to know when
the inferior's arch was set.  A subsequent patch in this series also
adds more things in set_arch.

Change-Id: I0005bd1ef4cd6b612af501201cec44e457998eec
Reviewed-By: John Baldwin <jhb@FreeBSD.org>
Approved-By: Andrew Burgess <aburgess@redhat.com>

gdb/python/python.c

@@ -1375,7 +1375,7 @@ gdbpy_format_address (PyObject *self, PyObject *args, PyObject *kw)
       /* Grab both of these from the current inferior, and its associated
 	 default architecture.  */
       pspace = current_inferior ()->pspace;
-      gdbarch = current_inferior ()->gdbarch;
+      gdbarch = current_inferior ()->arch ();
     }
   else if (arch_obj == nullptr || pspace_obj == nullptr)
     {