[M1] scripts/run-qemu.sh: QEMU x86_64 machine invocation

[mitsuakki]

Wrapper for qemu-system-x86_64 -kernel kernel.elf (Multiboot2 lets QEMU boot the ELF directly) or -drive with a raw disk image if using a custom boot sector. Add a -s -S variant for GDB attach and -serial stdio if using serial output.

Resources:

• QEMU x86_64 system emulation docs
• OSDev wiki: Bare Bones (Multiboot2 + QEMU) — end-to-end reference for this exact step