refactor: extract x86 CMOS I/O from drivers/rtc.c to HAL layer

refactor: extract x86 CMOS I/O from drivers/rtc.c to HAL layer

Created include/hal/rtc.h with generic HAL RTC interface and
src/hal/x86/rtc.c with x86 CMOS port I/O implementation.

src/drivers/rtc.c is now fully architecture-agnostic: it calls
hal_rtc_read_raw() for hardware access and keeps only the generic
BCD-to-binary conversion and UNIX timestamp calculation logic.

This follows the same HAL pattern used by timer, keyboard, uart,
and video drivers.