Issue an internal warning on first deprecated function call

[Carlos O'Donell]

> 
> A variation on the above idea is to scan a newly created gdbarch
> struct to see if any deprecated functions are being used.  If so,
> print only one message per session.  (I.e, instead of seeing N
> messages from N deprecated function, you'd only see one message for
> the whole lot.) The nuisance factor could be cut back even further if
> we arrange for .gdbinit setting which can be used to disable such
> messages.

I like the 'once per session' idea.
Can we make the that message a rather largish banner?

c.