feat(kernel): added improved time management as well as RTC support

[FlareCoding]

Note

High Risk
Touches AArch64 trap vectors and scheduler switch/return semantics, which are highly sensitive and can cause hard-to-debug crashes if incorrect; RTC/syscall additions are comparatively straightforward but broaden kernel surface area.

Overview
Adds wall-clock time support by introducing an rtc subsystem (x86 CMOS RTC, AArch64/QEMU PL031, and an RPi4 build-epoch fallback via STLX_BUILD_EPOCH) and wiring it into boot so clock::boot_realtime_ns() can back realtime calculations.

Implements Linux-compatible time syscalls (clock_gettime, clock_getres, gettimeofday) and registers the syscall numbers for both x86_64 and aarch64; userland gains new date and clockbench apps to exercise these APIs.

Fixes an AArch64 context-switch/trap-return correctness issue by extending trap_frame with sp_el1 and rewriting the exception vector epilogue + scheduler handoff to properly switch EL1 exception stacks when returning to EL0t/EL1t versus EL1h, preventing kernel stack corruption across task switches.

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