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

include/arch/aarch64/cpu.h

@@ -0,0 +1,24 @@
+#ifndef _ARCH_AARCH64_CPU_H
+#define _ARCH_AARCH64_CPU_H
+
+#include <stdint.h>
+#include <stdbool.h>
+
+typedef struct {
+    uint32_t implementer;
+    uint32_t variant;
+    uint32_t architecture;
+    uint32_t part_num;
+    uint32_t revision;
+    char     model_name[64];
+    uint32_t current_el;
+    uint64_t core_frequency_hz;
+} aarch64_cpu_info_t;
+
+void aarch64_cpu_init(void);
+const aarch64_cpu_info_t* aarch64_get_cpu_info(void);
+uint32_t aarch64_get_current_el(void);
+uint64_t aarch64_get_midr(void);
+uint64_t aarch64_get_mpidr(void);
+
+#endif // _ARCH_AARCH64_CPU_H