Add minimal STM32F103C6 support (#17853)

Unfortunately, the crippled versions of “Bluepill” boards with
STM32F103C6xx chips instead of STM32F103C8xx are now sold all over the
place, sometimes advertised in a confusing way to make the difference
not noticeable until too late.  Add minimal support for these MCUs in
the common “Bluepill with stm32duino” configuration, so that it could be
possible to make something useful from those boards (although fitting
QMK into the available 24 KiB of flash may be rather hard).

(In fact, I'm not sure whether the “STM32” part of the chip name is
actually correct for those boards of uncertain origin, so the onekey
board name is `bluepill_f103c6`; another reason for that name is to
match the existing `blackpill_f401` and `blackpill_f411`.)

The EEPROM emulation support is not included on purpose, because
enabling it without having a working firmware size check would be
irresponsible with such flash size (the chance that someone would build
a firmware where the EEPROM backing store ends up overlapping some
firmware code is really high).  Other than that, enabling the EEPROM
emulation code is mostly trivial (the `wear_leveling` driver with the
`embedded_flash` backing store even works without any custom
configuration, although its code is significantly larger than the
`vendor` driver, which may also be important for such flash size).

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