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

mm/kmalloc.c

@@ -5,7 +5,7 @@
 #include <kernel/printk.h>
 
 #define HEAP_MAGIC 0x53554248 // "SUBH"
-#define HEAP_INITIAL_PAGES 1024 // 4 MB initial kernel heap
+#define HEAP_INITIAL_PAGES 4096 // 16 MB initial kernel heap
 
 typedef struct heap_block {
     uint32_t magic;