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

kernel/printk.c

@@ -45,10 +45,17 @@ int vprintk(const char* fmt, va_list args) {
     // 1. Output to active TTY console
     tty_write(display_ptr, (size_t)len);
 
-    // 2. Output to COM1 serial port for host debugging
+    // 2. Output to serial console (COM1 on x86, PL011 on AArch64)
+#if defined(__aarch64__)
+    extern void uart_pl011_putc(char c);
+    for (int i = 0; i < len; i++) {
+        uart_pl011_putc(display_ptr[i]);
+    }
+#else
     for (int i = 0; i < len; i++) {
         serial_write_char(display_ptr[i]);
     }
+#endif
 
     // 3. Store in kernel dmesg ring buffer
     dmesg_append(display_ptr, (size_t)len);