For SMP Boot Code: allocate physical memory instead

[sarahspberrypi]

Instead of forcefully writing the SMP boot code to physical address 0x8000, free physical memory is allocated.

This is fine because the application processor will install the boot processor's page table: https://github.com/hermit-os/kernel/blob/054d2d6dafed90a92fa72a9aa4559f1ab83d0d15/src/arch/x86_64/kernel/apic.rs#L719-L721