gpio: move varargs hack outside #ifdef GPIOLIB

gpio: move varargs hack outside #ifdef GPIOLIB

commit 5767dae43dee06da134e4bad30e44d064103eab8
"gpio: add flags argument to gpiod_get*() functions"
added a dynamic flags argument to all the GPIOD getter
functions, however this did not cover the stubs so
when people used gpiod stubs to compile out descriptor
code, compilation failed.

Solve this by:
- Also rename all the stub functions __gpiod_*
- Moving the vararg hack outside of #ifdef CONFIG_GPIOLIB
  so these will always be available.

Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>