Add support for exception handling with multiple versions of

        the Ada runtime:
        * ada-lang.c: Update general comments on how Ada exception catchpoints
        are implemented.
        (raise_sym_name, raise_unhandled_sym_name, raise_assert_sym_name)
        (__gnat_raise_nodefer_with_msg): Delete.
        (ada_unhandled_exception_name_addr_ftype): New type.
        (exception_support_info): New type.
        (ada_unhandled_exception_name_addr): Add forward declaration.
        (ada_unhandled_exception_name_addr_from_raise): Likewise.
        (default_exception_support_info): New constant.
        (exception_support_info_fallback): Likewise.
        (exception_info): New global variable.
        (ada_exception_support_info_sniffer): New function.
        (ada_executable_changed_observer): Likewise.
        (ada_unhandled_exception_name_addr_from_raise): Renamed from
        ada_unhandled_exception_name_addr.
        (ada_unhandled_exception_name_addr): Reimplement to match the
        latest Ada runtime implementation.
        (error_breakpoint_runtime_sym_not_found): Delete.
        (ada_exception_sym_name): Get the exception sym name from
        exception_info rather than hardcoding it.
        (ada_exception_sal): Add call to ada_exception_support_info_sniffer.
        Update error handling.
        * Makefile.in (ada-lang.o): Add dependency on observer.h.