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/linker.ld

@@ -0,0 +1,45 @@
+OUTPUT_FORMAT("elf64-littleaarch64")
+OUTPUT_ARCH(aarch64)
+ENTRY(_start)
+
+SECTIONS
+{
+    . = 0x40080000;
+
+    __kernel_start = .;
+
+    .text.boot : {
+        KEEP(*(.text.boot))
+    }
+
+    .vectors : ALIGN(2048) {
+        KEEP(*(.vectors))
+    }
+
+    .text : ALIGN(4096) {
+        *(.text .text.*)
+    }
+
+    .rodata : ALIGN(4096) {
+        *(.rodata .rodata.*)
+    }
+
+    .data : ALIGN(4096) {
+        *(.data .data.*)
+    }
+
+    .bss : ALIGN(4096) {
+        __bss_start = .;
+        *(.bss .bss.*)
+        *(COMMON)
+        . = ALIGN(4096);
+        __bss_end = .;
+    }
+
+    .stack : ALIGN(4096) {
+        . += 0x20000; /* 128 KB dedicated kernel stack */
+        stack_top = .;
+    }
+
+    __kernel_end = .;
+}