Fix a couple libstdc++ pretty-printer bugs

[Tom Tromey]

>>>>> "Benjamin" == Benjamin Kosnik <bkoz@redhat.com> writes:

Benjamin> Sorry, thought I had already replied to this.

No problem.

Benjamin> Ok, of course. FYI I consider you the maintainer for this
Benjamin> functionality, notice is certainly appreciated and in your
Benjamin> usual good form but you should feel free to check in at will
Benjamin> for items to do with pretty printing.

Thanks.  I am going to commit this patch plus the relocatabilty patch
soon.

One more patch is nearing completion (better printing for
std::string), and we also recently discovered a fairly severe design
flaw in our approach to MI that will require a reasonably substantial
change.

Tom