[kernel][x86] Add Multiboot trampoline loader

This adds a small Multiboot-compliant "kernel" that takes
a complete x64 ZBI as its "module" (aka initrd or ramdisk).
It simulates a ZBI-compatible boot loader and synthesizes
ZBI data items from the Multiboot information.

Bug: ZX-2225
Test: manual boot in qemu
Change-Id: I39ef207c527ddd2fc8ffc6b01b7ea93d683fd756