Add AT32 support

[rhgndf]

Describe the PR
This makes it work on AT32F403A. Tested with the WeAct AT32F403A Blackpill board.

Additional context
https://github.com/WeActStudio/WeActStudio.BlackPill

[rhgndf]

Build works with:

cdc_msc$ make BOARD=at32f403a_weact_blackpill all

Memory region         Used Size  Region Size  %age Used
           FLASH:       23104 B         1 MB      2.20%
             RAM:       11816 B        96 KB     12.02%
CREATE _build/at32f403a_weact_blackpill/cdc_msc.bin
arm-none-eabi-objcopy -O binary  _build/at32f403a_weact_blackpill/cdc_msc.elf _build/at32f403a_weact_blackpill/cdc_msc.bin
CREATE _build/at32f403a_weact_blackpill/cdc_msc.hex

   text    data     bss     dec     hex filename
  14784    8260    3568   26612    67f4 _build/at32f403a_weact_blackpill/cdc_msc.elf

The board works with a usb serial and a mass storage device being detected.

[hathach]

thank you, I ordered an at32f403 board, will test this out when it is delivered.