Add x86_64 architecture support with complete 64-bit kernel implement…

[hamzasiddiqui2830-art]

…ation

• src/kernel/arch/x86_64/arch.zig: New 64-bit architecture backend with CPU state, GDT/TSS, IDT, paging, and I/O operations
• src/kernel/arch/x86_64/boot.asm: Assembly bootloader with Multiboot2 support and long mode entry
• src/kernel/arch/x86_64/gdt.zig: 64-bit Global Descriptor Table with kernel/user code/data segments and TSS
• src/kernel/arch/x86_64/idt.zig: 64-bit Interrupt Descriptor Table with interrupt/exception handling
• src/kernel/arch/x86_64/interrupt_stubs.asm: Assembly interrupt/IRQ stubs with register preservation
• src/kernel/arch/x86_64/irq.zig: IRQ handling system with PIC remapping and handler registration
• src/kernel/arch/x86_64/isr.zig: Exception handling with detailed CPU state reporting
• src/kernel/arch/x86_64/keyboard.zig: PS/2 keyboard driver with scancode-to-ASCII conversion
• src/kernel/arch/x86_64/link.ld: Linker script defining 64-bit kernel memory layout and sections
• src/kernel/arch/x86_64/multiboot.zig: Multiboot2 header definitions and information parsing
• src/kernel/arch/x86_64/paging.zig: 4-level page table implementation with virtual/physical address mapping
• src/kernel/arch/x86_64/pci.zig: PCI device enumeration and configuration space access
• src/kernel/arch/x86_64/pit.zig: Programmable Interval Timer initialization and frequency control
• src/kernel/arch/x86_64/rtc.zig: Real Time Clock date/time retrieval with BCD conversion
• src/kernel/arch/x86_64/serial.zig: Serial port communication driver with configurable baud rates
• src/kernel/arch/x86_64/syscalls.zig: Syscall handler framework with MSR setup for fast system calls
• build.zig: Updated build system to support x86_64 target with corresponding QEMU commands
• .gitignore: Expanded ignore patterns for build artifacts and editor files

The implementation provides a complete 64-bit kernel foundation with memory management, interrupt handling, device drivers, and user mode transition capabilities. Runtime tests validate the 64-bit kernel functionality including user task state and memory management operations.