feat(arch): Add AArch64 / ARMv8i CPU support, cross-compilation & Linux Kconfig TUI

- Implement AArch64 HAL, vector table (16 aligned vectors), GICv2, Generic Timer, Stage-1 MMU (39-bit VA), PL011 UART
- Build unified multi-architecture abstraction layer in include/arch/arch.h
- Create interactive Linux-like Kconfig TUI menuconfig in scripts/kconfig/menuconfig.py
- Support make menuconfig, defconfig, x86_64_defconfig, aarch64_defconfig
- Update master Makefile for cross-compiling x86_64 (x86_64-elf-) and AArch64 (aarch64-linux-gnu-)
- Dynamic architecture awareness across printk, uname, neofetch, and LazyBox

arch/aarch64/cpu/timer.c

@@ -0,0 +1,49 @@
+#include <arch/aarch64/timer.h>
+#include <arch/aarch64/gic.h>
+#include <kernel/timer.h>
+#include <kernel/printk.h>
+
+static uint64_t timer_ticks = 0;
+static uint64_t timer_freq = 62500000; // Standard 62.5MHz QEMU virt frequency
+static uint32_t tick_interval = 0;
+
+void aarch64_timer_handle_irq(void) {
+    timer_ticks++;
+
+    // Reset next comparator interval
+    __asm__ volatile("msr cntv_tval_el0, %0" :: "r"((uint64_t)tick_interval));
+
+    // Call generic kernel timer tick
+    timer_tick();
+}
+
+static void gic_timer_wrapper(uint32_t irq) {
+    (void)irq;
+    aarch64_timer_handle_irq();
+}
+
+void aarch64_timer_init(uint32_t freq_hz) {
+    if (freq_hz == 0) freq_hz = 100;
+
+    __asm__ volatile("mrs %0, cntfrq_el0" : "=r"(timer_freq));
+    tick_interval = (uint32_t)(timer_freq / freq_hz);
+
+    // Register with GIC (PPI 27 = Virtual Timer IRQ)
+    gic_register_handler(ARCH_TIMER_VIRT_IRQ, gic_timer_wrapper);
+
+    // Set countdown and enable virtual timer
+    __asm__ volatile("msr cntv_tval_el0, %0" :: "r"((uint64_t)tick_interval));
+    uint64_t ctl = 1; // Enable=1, IMASK=0
+    __asm__ volatile("msr cntv_ctl_el0, %0" :: "r"(ctl));
+
+    printk(KERN_INFO "Timer: ARM Generic Arch Timer active (%u Hz, Base Clock: %llu MHz)\n",
+           freq_hz, timer_freq / 1000000);
+}
+
+uint64_t aarch64_timer_get_ticks(void) {
+    return timer_ticks;
+}
+
+uint64_t aarch64_timer_get_freq(void) {
+    return timer_freq;
+}