Support GEM outside Arduino framework

[mrv96]

Hi, I would like to share how I successfully ported GEM to STM32 environment.

I know that this PR is rough, mainly because hardcodes stm32u5xx_hal.h, but this issue can be easily solved using a function which registers a callback for delays.

If someone is interested in this feature, with some refinement, I think this PR could become mergeable.

I tried this with U8g2.

P.S. this breaks backward compatibility because by default U8G2 class is not printable outside Arduino framework

[Spirik]

Hi, @mrv96 ! That's quite an undertaking! Thank you for sharing=)

I have zero experience working with STM32 (so I can not validate it right away), but I think even in its current state this PR can be very valuable to anyone, who would like to use GEM there. I will add link to it somewhere in wiki, so that it would be more easily discoverable.

Thank you very much for your contribution!