devel/gdb: Handle recent 14.0 systems with libc++ in /lib.

The auto-load python wrapper for libc++ pretty printers has to be
installed at the same path as libc++ to work correctly. Update the
port to install the top-level wrapper in lib/ under the auto-load data
directory instead of usr/lib on recent 14.0 systems.

Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D34636