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

net/core/net.c

@@ -5,7 +5,7 @@
 #include <lib/string.h>
 #include <lib/printf.h>
 #include <mm/kmalloc.h>
-#include <arch/x86_64/pit.h>
+#include <arch/arch.h>
 #include <kernel/printk.h>
 
 #define MAX_ARP_ENTRIES 16
@@ -288,7 +288,7 @@ int net_ping(uint32_t target_ip, uint32_t count, uint32_t timeout_ms) {
         if (wait_ticks == 0) wait_ticks = 100;
 
         while (!ping_received && (pit_get_ticks() - t_start < wait_ticks)) {
-            __asm__ volatile ("hlt");
+            arch_halt();
         }
 
         if (ping_received && ping_id == p_id && ping_seq == seq) {