selftests: gpio: restore CFLAGS options

All the CFLAGS options were incorrectly removed in the recent rework
of the GPIO selftests.  While some of the flags were specific to the old
implementation the remainder are still relevant.  Restore those options.

Signed-off-by: Kent Gibson <warthog618@gmail.com>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>