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/x86_64/cpu/arch.c

@@ -0,0 +1,32 @@
+#include <arch/arch.h>
+#include <arch/x86_64/gdt.h>
+#include <arch/x86_64/idt.h>
+#include <arch/x86_64/isr.h>
+#include <arch/x86_64/pic.h>
+#include <arch/x86_64/pit.h>
+#include <arch/x86_64/cpuid.h>
+#include <kernel/timer.h>
+
+void arch_early_init(void) {
+    // x86_64 serial and tty initialized in main
+}
+
+void arch_init(void) {
+    gdt_init();
+    idt_init();
+    isr_init();
+    pic_init();
+    pit_init(100);
+    timer_wheel_init();
+    arch_enable_interrupts();
+}
+
+const char* arch_get_name(void) {
+    return "x86_64 (AMD64)";
+}
+
+uint32_t arch_get_cpu_id(void) {
+    uint32_t eax = 0, ebx = 0, ecx = 0, edx = 0;
+    cpuid(1, &eax, &ebx, &ecx, &edx);
+    return (ebx >> 24) & 0xFF;
+}