[M1] Write linker script for kernel load address

[mitsuakki]

Define memory layout for the kernel ELF: entry point, section placement, and where the higher-half or identity-mapped kernel gets linked. If using GRUB/Multiboot2, must also place a Multiboot2 header early in .text. If writing your own boot sector, the address depends on where your stage 2 loads the kernel in memory.

Resources:

• OSDev wiki: Linker Scripts
• OSDev wiki: Higher Half Kernel — decide now if you're doing this in M1 or M3