ChibiOS timer fixes #16017

Extract the code which effectively makes a 32-bit tick counter from a possibly 16-bit ChibiOS system timer into a separate function. Does not really change the behavior of the timer API, but makes the actions done in `timer_clear()` and `timer_read32()` more obvious.

The QMK timer API implementation for ChibiOS used a 32-bit tick counter (obtained from the ChibiOS system timer) and then converted the value to milliseconds to produce the timer value for QMK. However, the frequency of the ChibiOS timer is above 1000 Hz in most cases (values of 10000 Hz or even 100000 Hz are typically used), and therefore the 32-bit tick counter was overflowing and wrapping around much earlier than expected (after about 5 days for 10000 Hz, or about 12 hours for 100000 Hz). When this wraparound happened, the QMK timer value was jumping back to zero, which broke various code dealing with timers (e.g., deferred executors). Just making the tick counter 64-bit to avoid the overflow is not a good solution, because the ChibiOS code which performs the conversion from ticks to milliseconds may encounter overflows when handling a 64-bit value. Adjusting just the value converted to milliseconds to account for lost 2**32 ticks is also not possible, because 2**32 ticks may not correspond to an integer number of milliseconds. Therefore the tick counter overflow is handled as follows: - A reasonably large number of ticks (the highest multiple of the ChibiOS timer frequency that fits into uint32_t) is subtracted from the tick counter, so that its value is again brought below 2**32. The subtracted value is chosen so that it would correspond to an integer number of seconds, therefore it could be converted to milliseconds without any loss of precision. - The equivalent number of milliseconds is then added to the converted QMK timer value, so that the QMK timer continues to count milliseconds as it was before the tick counter overflow.

…reliable The code which extends the 16-bit ChibiOS system timer to a 32-bit tick counter requires that it is called at least once for every overflow of the system timer (otherwise the tick counter can skip one or more overflow periods). Normally this requirement is satisfied just from various parts of QMK code reading the current timer value; however, in some rare circumstances the QMK code may be blocked waiting for some event, and when this situation is combined with having a rather high timer frequency, this may result in improper timekeeping. Enhance the timer reliability by adding a ChibiOS virtual timer which invokes a callback every half of the timer overflow period. The virtual timer callback can be invoked even when the normal QMK code is blocked; the only requirement is that the timer interrupts are enabled, and the ChibiOS kernel is not locked for an excessive time (but the timer update will eventually work correctly if the virtual timer handling is not delayed by more than a half of the timer overflow period). Keeping a virtual timer always active also works around a ChibiOS bug that can manifest with a 16-bit system timer and a relatively high timer frequency: when all active virtual timers have delays longer than the timer overflow period, the handling of virtual timers stops completely. In QMK this bug can result in a `wait_ms()` call with a delay larger than the timer overflow period just hanging indefinitely. However, when the timer update code adds a virtual timer with a shorter delay, all other virtual timers are also handled properly.

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ChibiOS timer fixes #16017

ChibiOS timer fixes #16017

Commits on Jan 23, 2022