Change solib-aix.c to use type-safe registry

Change solib-aix.c to use type-safe registry

This changes solib-aix.c to use the type-safe registry, and removes a
use of VEC in the process.

gdb/ChangeLog
2019-07-10  Tom Tromey  <tromey@adacore.com>

	* solib-aix.c (lm_info_aix_p): Remove typedef.  Don't define VEC.
	(struct solib_aix_inferior_data) <library_list>: Change type.
	(solib_aix_inferior_data_handle): Change type.
	(get_solib_aix_inferior_data): Update.
	(solib_aix_free_library_list): Remove.
	(library_list_start_library): Update.
	(solib_aix_parse_libraries, solib_aix_get_library_list): Change
	return type.
	(solib_aix_get_library_list)
	(solib_aix_solib_create_inferior_hook, solib_aix_current_sos)
	(solib_aix_normal_stop_observer, _initialize_solib_aix): Update.