gpio: move varargs hack outside #ifdef GPIOLIB

commit 39b2bbe3
"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>