Convert ada_add_global_exceptions

This converts ada_add_global_exceptions to the callback approach,
merging the search loop and the call to expand_symtabs_matching.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=16994
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=16998
Acked-By: Simon Marchi <simon.marchi@efficios.com>