RP2040's Pico SDK provides time_us_64() which reads the hardware
timer registers directly, returning native uint64_t microseconds.
This eliminates the need for 32-bit rollover tracking on this
platform.
millis() is now derived from millis_64() (like Zephyr) instead of
the other way around, unifying the time source.
Also removes IRAM_ATTR from all RP2040 HAL functions. On RP2040,
IRAM_ATTR is a no-op: the functions it decorated (millis, micros,
delayMicroseconds, arch_get_cpu_cycle_count) all call into Pico SDK
functions that reside in flash (XIP), so the attribute had no effect.
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