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/uart.h

@@ -0,0 +1,34 @@
+#ifndef _ARCH_AARCH64_UART_H
+#define _ARCH_AARCH64_UART_H
+
+#include <stdint.h>
+#include <stddef.h>
+
+#define PL011_UART0_BASE 0x09000000
+
+// PL011 Registers
+#define UARTDR   0x000
+#define UARTRSR  0x004
+#define UARTFR   0x018
+#define UARTILPR 0x020
+#define UARTIBRD 0x024
+#define UARTFBRD 0x028
+#define UARTLCR  0x02C
+#define UARTCR   0x030
+#define UARTIFLS 0x034
+#define UARTIMSC 0x038
+#define UARTRIS  0x03C
+#define UARTMIS  0x040
+#define UARTICR  0x044
+
+// Flag register bits
+#define UARTFR_TXFF (1 << 5)
+#define UARTFR_RXFE (1 << 4)
+
+void uart_pl011_init(void);
+void uart_pl011_putc(char c);
+void uart_pl011_puts(const char* str);
+char uart_pl011_getc(void);
+int  uart_pl011_has_data(void);
+
+#endif // _ARCH_AARCH64_UART_H