GCC support

[wavexx]

This is based on #2 + #9, which works on current main branch with gcc-arm-noeabi 13.2.1.
Includes a few tweaks to fix the status bar as well (mentioned on #2).
Everything works as expected as far as I can see.

Memory region         Used Size  Region Size  %age Used
             RAM:        2176 B         4 KB     53.12%
           FLASH:       11680 B        16 KB     71.29%
          EEPROM:          0 GB        448 B      0.00%

Special thanks to @bmigette for pyocd/pyOCD#1646 which got me going.

[surfdado]

Hi, thanks for all this - Lukash told me about you. Could you please join the Floatwheel Discord if you haven't already? There's a lcm-development channel. There's a few others who have been trying their luck with gcc support, I'd like them to try your changes.

[surfdado]

void main(void) doesn't compile with Keil Studio so I changed it back to int main(void and added a return 0; at the end

[wavexx]

Sorry I was absent for a bit, but reporting my findings:

• works correct, for the most part
• it looks like there are some issues with ADC sampling. Occasionally the read voltages seem to be off, resulting in the status bar generally showing one red led only. Since this happens occasionally while riding, it's kind of annoying to debug
• for the same reason, it can occasionally be seen while charging (at the end of the charging cycle)
• I switched back to the original keil build for testing, and after a few weeks I noticed the original build also does the same, albeit much more rarely.

Pure guesswork so far, but I suspect there's some timing/isr issue already, and the gcc build just makes it more apparent.

[wavexx]

Fixed the main declaration and rebased. Untested (for the moment).