Makefile: add GNU global tags support

Makefile: add GNU global tags support

GNU Global is another tags engine which is more like cscope in being
able to support finding both references and definitions. You will be
un-surprised to know it also integrates well with Emacs.

The main benefit of integrating it into find-src-path is it takes less
time to rebuild the database from scratch when you have a lot of build
directories under your source tree.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20210114165730.31607-3-alex.bennee@linaro.org>