[RFA] Introduce solib_loaded observer

[Andrew Cagney]

> +@deftypefun void solib_loaded (struct so_list *@var{solib})
> +The specified shared library has been discovered to be loaded.  Note
> +that when @value{GDBN} calls this observer, the library's symbols
> +probably haven't been loaded yet.
> +@end deftypefun
> +
>  @deftypefun void solib_unloaded (struct so_list *@var{solib})
>  The specified shared library has been discovered to be unloaded.
>  @end deftypefun

This is good.

Andrew