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/configs/defconfig

@@ -0,0 +1,36 @@
+#
+# Automatically generated by SUB-OS Kconfig for aarch64
+#
+CONFIG_ARCH=aarch64
+CONFIG_ARCH_AARCH64=y
+# CONFIG_ARCH_X86_64 is not set
+CONFIG_BPF_VM=y
+CONFIG_DRV_CANVAS_2D=y
+CONFIG_DRV_PTY=y
+CONFIG_DRV_RAMDISK=y
+CONFIG_DRV_VIRTIO_BLK=y
+CONFIG_DRV_VIRTIO_NET=y
+CONFIG_DRV_VIRTIO_RNG=y
+CONFIG_FS_DEVFS=y
+CONFIG_FS_EXT2=y
+CONFIG_FS_FAT32=y
+CONFIG_FS_PROCFS=y
+CONFIG_FS_RAMFS=y
+CONFIG_FS_SYSFS=y
+CONFIG_FS_VFS=y
+CONFIG_IO_URING=y
+CONFIG_MM_PMM=y
+CONFIG_MM_SLAB=y
+CONFIG_MM_VMA=y
+CONFIG_NET_FILTER=y
+CONFIG_NET_HTTPD=y
+CONFIG_NET_SSHD=y
+CONFIG_NET_STACK=y
+CONFIG_OPTIMIZATION="-O2"
+CONFIG_PREEMPT=y
+CONFIG_SECURITY_KEYRING=y
+CONFIG_SECURITY_LSM=y
+CONFIG_SMP=y
+CONFIG_SUBOS_HOSTNAME="sub-os"
+CONFIG_SUBOS_VERSION="0.2.0-lts"
+CONFIG_USERLAND_LAZYBOX=y