um: Enable FORTIFY_SOURCE

Enable FORTIFY_SOURCE so running Kunit tests can test fortified
functions.

Signed-off-by: Kees Cook <keescook@chromium.org>
Tested-by: David Gow <davidgow@google.com>
Link: https://lore.kernel.org/r/20220210003224.773957-1-keescook@chromium.org